RCFREEK:

Definately use 10 - 15% nitro in your 50SX.

Ultrathrust muffler from Performance Specialties (pspec.com) should give you a least 750 more RPM.
$64.95 for one to fit OS 50SX.

APC props will also help - 10X6, 10X7, 11X6... you may have to experiment with prop sizes

Go to NSRCA.org, where I believe they have a flight trimming guide - use it to help trim out your airplane before
adding any elevator to rudder mix in your transmitter - which may be neccessary anyway.

The smaller planes are never gonna be perfect, but you can still have fun with them and learn alot.

I have seen Zen 50's fly before and they do quite well - but you have to keep the speed up a little.