Start with 0.25's for outdoors...you'll drop about 30fps from 0.20's.
Try a bag of 0.28's...the drop from 0.25's is not too bad.
(for the anal retentive nit-picks, there's some generalizations below to get the point across...your personal results may vary)
It's not about maximum range...it's about your effective range. At the absolute furthest you can shoot with 0.20's (i.e. before your BB's hit the ground in front of your target and they laugh at you), your "group" of shots will be about the size of person...not the best for reliable shooting/hits. Imagine having to launch a 30rnd burst at someone just to hit them...your mags won't last very long that way. To get more reliable hits....you have to get closer...until you can reliably hit someone with 1-2 shots. With 0.20's, this will be fairly close.
Now switch to 0.25's...almost like magic at your groups will shrink. Instead of shooting "minute of man" you're shooting "minute of chest". The absolute furthest might be a bit closer...but instead of launching 30 shots to get a hit, you're shooting a short 5rnd burst to get a hit. To reliably hit someone with 1-2 shots...you don't need to move up as close as with 0.20's. Although your maximum absolute range may have decreased a bit, your maximum effective range has greatly improved.
With 0.28's and 0.30's...your shots will go straighter and your hits will be more predictable. They carry more energy when they hit...so you opponent will feel/hear your shot better and call their hit. Extreme long range hits from light bbs feel like nothing and with a lot of gear you may not even hear it.
So why not use the heaviest BBs you can get? Because your range does decrease and the bbs are slowing down. There's a 60-70fps drop between 0.20s and 0.30s...and if you were shooting 0.43s your bbs would be going really slowly (i.e. if someone saw you shooting at them...they could "matrix" doge the shot without too much effort (falling to the ground is easy).
Best of luck