Slip it all day long. It's not a Cessna.
The IO-360 has it's own pecularities & not a lot of people take the time to figure them out. It's worth it though, sweet little engine until you get to rebuild it. Butch is right (Hey Butch! ) There's a few rules here. Don't baby it, run it at 23-24 squared like James said (Hey James! ) Make damn sure your baffling is good. This engine doesn't suffer overheating well at all and was the reason it suffered in the rear spot of the 337 to begin with. There's a nasty off idle stumble that'll add some gray hairs to your head if your'e not warry of it on short final. Sorry, no experience with GAMI's, herard they're great, willing to accept sponsorship though....
Flight: We covered slips, the M-5 wings are the same length, have the ailerons moved out board and about 18" more flap. The big advantage is that most -5 wings have the aux tanks which I can't really see myself ever being able to without here. (I have the -4 wings with aux tanks) The -4 slips great, arguably has a better crosswind component and is ~100-120# lighter than the -5, -6 & -7's. They are well balanced on the controls and with experience, will do some pretty amazing things.
I have 31's and cruise ~125 or so. The gap seal is the best $50 you'll ever spend. The VG's won't lower your stall much if at all but will give you a tremendous increase in aileron authority. Worth it in my book if you spend much time on the backside of the power curve. Oh yeah, spend a lot of time in this region at altitude, get real familar with it. It will keep you alive.
Your best mod is a lot of gas. Have fun with it