Swapping springs all the time is a pain in the ass. A well tuned gun (good barrel, hop-up, nozzel, etc.) will shoot fine in the 300-340 range. Stock Classic Army guns shoot about 315 and are great out-of-the-box guns.
Here's a few other options, if you're really bent on having higher outdoor FPS and don't want 2 guns:
-A P90 with 2 mechboxes. A JG P90 is about $350, and you can grab another 100% complete V6 mechbox for around $40-$70, and have an "indoor" and "outdoor" mechbox. Swapping a P90 mechbox is simple as hell -remove 2 screws, remove backplate, swap mechbox. Literally takes 2 minutes and you're not opening a mechbox or fiddling with springs. A P90 is a great outdoor gun. I've played outdoor with a stock TM P90 before (~280 fps), worked like a charm.
-ICS L85: A little big for indoor perhaps, but has an adjustable spring guide. Again, no need to open mechbox. On the lowest FPS setting it shoots around 340 IIRC stock. Swap in a different spring and you can get that down.
-Man up and play with a stock Tokyo Marui or Classic Army gun. Playing outdoor with a stock gun will make you a better player in the end anyway.
Just my $0.02
"Someone in a Prius tried to race me at a stop sign the other day. I couldn't believe it. I had him for the first 100 feet or so but I can only walk so fast."