I had the same issues with my Avistar. Flew it with the OS40LA (flies fine, but not peppy off the runway). Swapped out the 40La for a Magnum 46XLS. Big difference, but much heavier. Ended up moving the battery back behind the servo tray to get the balance right. It'll still fly ok with the 46 on the nose and the battery in the same place as with the 40LA, but it'll feel sluggish getting off the ground and won't do any really fast maneuvers. One of the joys of trainers is their large acceptable CG range. With the Magnum 46XLS on an 11x5 APC mine has great vertical, but is just short of hover.

Once you move the battery back to get the CG farther to the rear, you'll be surpised as this plane will do all kind of crazy stuff and recovery is a breeze. On mine if I fly about 1/2 throtle and pull full rudder and elevator, I get a great snap. If I hold the rudder/elevator and firewall the throttle as the snap starts, it goes from the snap into a short climbing inverted spin (3-5 revolutions) and then falls out into regular spin. If I go to 1/2 power or less as it falls out of the inverted spin, it does a nice spinning dive. To recover from any of these, all I have to do is go to half throttle or less and center the sticks and it settles down within 1 to 2 seconds in almost every case. Then you just pull out and do it again and again and again and again....

Side note, mine has a solid 1/4" metal rod connecting the wing halves and I use 8 rubber bands on a side, which I cut off at the end of the day. These maneuvers will likely spell instant death for an Avistar that doesn't have the wing center section reinforced in some way. Don't say i didn't warn ya! I lifted the wing leading edge one time pulling out of a power dive with 6 bands per side and it wasn't pretty, though I managed to get the plane down on the ground.

Duke