I would recommend using a Pro Zinger 15x4 on the Saito 82 for UCD 40. I tached this combo on 20/20 fuel and it peaked at 9600 rpm. Static thrust is about 9+ lb, as experienced by my uncalibrated arm.

I do know that this engine will swing APC 15x4W at about the same RPM, but will have greater thrust, maybe 10+ lb. But I am not sure about the spool up. The 15x4W is a little heavy for even the Saito 91 to spool up quickly.

For reference, Saito 91 pulling APC 15x4W @ 10200 rpm measured 11 lb. static thrust.
Saito 72 pulling APC 14x4W @ 9800 rpm measured 8.5 lb. static thrust.

Keep your plane under 5.5 lb, and it will be great combo.