I've built three retract planes, all nitro, with two being twins. Flown with gear down and up.
To me, the difference was not huge - minor difference. Gear up seemed a bit more consistent
between inside and outside, but with the gear down, still could fly about the same.
Wow factor was much more noticeable.
With electric, gear up = less drag = less battery usage. Go back and look at Bob Hunt's twin
postings to get an idea.
Pics - Sheek's Mosquito w/retracts (2) Thunder Tiger 25s
My own design twin w/ retracts (2) 28s