I put an RM06 on my G29 a couple of weeks ago and have run a variety of loads including some 200 gr. WFN handloads that chrono between 1120 and 1250 fps.  I have had no issues at all and overall am pleased with the setup.  I used a "Sightmount" http://www.sight-mount.com/ that is a rear sight as well as a picatinny mount, rather than milling the slide.  It gives me a good buis in the event the red dot fails, but the more I use it the more confidence I have in it.  I just wish I had done it sooner because it is much easier to do precision shooting with my old eyes.

I have had no trouble picking up the red dot even though it is not as sharp as it could be due to my astigmatism.  I can still shoot better with it than trying to focus on iron sights.  It's especially good in low light that is not dark enough for tritium sights to show, yet too dark for standard iron sights - like for hunting, which is what I originally wanted it for.

I'm not sure how it would work with a Streamlight because when I am out in the evening with my handheld flashlight it's easier to see the red dot if I hold the light above my head.  The brightest setting is pretty bright, though, so possibly if I could come to my usual 2 hand hold it would pick up more readily than trying to hold a light in one hand and the gun in the other.

I may eventually decide to mill the slide, but so far it is really pretty handy as is, and rugged.  I use it for a charging handle and it works great.

You don't say what does not fit you with the Glocks.  I have a small hand so I had to go to the SF model in both G20 and G29.  I tend to shoot the G29 better because the G20 is still a little bit big(long in the handle).  If you handload it is really tough to beat the G29 with a KKM 4.6" barrel.  Mine handles full power loads well.  I get 1240 fps with 200 gr WFN cast using AA#9, no pressure problems and brass is re-usable.  For medium loads Longshot is very good for about 1150 fps, same bullet and shoots in a 1 1/2" circle at 22 yds.

With the standard length barrel I lose about 50 fps.  I installed a "Sightmount" rear sight/picatinny rail with a Trijicon RMR 06 RDS - makes it easier on my 69 year old eyes, but it is a very compact package for EDC with the standard length barrel or woods carry with the 4.6" barrel.  The G20 has the additional option of a 6" KKM for more of a hunting application and probably handles the heavy loads at least as well as if not better than the G29 - but it is a bit bigger to  pack around.

I haven't tried to  push the envelope with the hot loads.  I'm quite happy with 200 gr at 1240 fps for anything I want to tackle, but there might be room for a little more velocity if a person is inclined in that direction.  If an extra 50 fps is your goal, I would lean toward the G20 with a 6"KKM.

Sometime I would like to get my hands on an EEA, but I doubt that anything is going to come along and replace my Glock 29.  The G29 would complement your G20 very nicely and it is hard to compete with the availability of after-market accessories for Glocks

I tried a LW barrel in my Gen 3 -29 but still had issues with brass not being re-loadable due to lack of adequate support.  I went to KKM and have been pleased.  I have a stock length KKM, a 4.6" KKM and a stock 40S&W conversion.  I also mounted a Sight Mount rear sight/picatinny rail on one of my G29s(Gen3) that holds a Trijicon RMR 06 RDS.  That is especially good for testing loads because I can shoot it more accurately than open sights.  It also has the advantage of easy adjustment for sight tuning when switching barrels.

With the 4.6" barrel and the RDS it is like a compact G20 that shoots 200 grain WFN cast at 1250 fps for woods carry and hunting.  My Gen4 is set up with the tactical adjustable rear and a Dawson Precision front for EDC.  The 40 conversion barrel shoots closely enough to POA with the 10mm barrel that I can switch to either barrel depending on ammo availability.  I like the .40 mainly for the abundance of brass.  I can seat the bullets longer than the standard .40 seating depth which enables me to safely achieve velocities of around 1140 with the 200 grain WFN - great for general purpose and practice as well as SD.

I use the Wolff non-capture 21 lb spring and rod in the Gen 3 and just have the OEM in the Gen 4 since I don't shoot max loads for EDC.  I agree with The Shadow's assessment of the 23lb spring. I've tried it.
The Gen3 has a 3.5 lb connector, but the Gen4 is stock, although I did the polish job on the internal parts and it is almost as good as the Gen3.

I have a Gen3 G20 also, but I prefer the size of the G29 for carrying, plus my hands are small so I shoot it better.

Incidentally, I have gone to powder coating for my cast bullets and am quite happy with the results - much cleaner and easier to handle, no leading, good accuracy.  I've never shot jacketed in any of my 10's.

Much as I like my G29's (I have 2 of them), I wouldn't want to get rid of my G20 and only have the G29, even though I do have a KKM 4.6" barrel for the 29, so it shoots my hot loads the same velocity as the G20.  I admit that the 29 is much preferable for EDC(I never even know I have it on most of the time) but my general philosophy has been to just add another gun rather than trade.  One of these days you might get one of the Double D outfits and then you will be glad you still have the G20.  I've been thinking I might get a 6" KKM barrel for my 20 and then I would definitely want the Double D chest holster.  I think you need one of each!

I also have a G20 and 2 G29's, one of which is a Gen 4. The others are both Gen 3 SF's. Can you tell I like G29's?
Snuffy, you probably need a Gen 4 29, although there is really not much difference between it and the Gen3 SF.