I always ran a blend of Syth and castor - Shims are dependant on run desired and switching. anythign up to 15% nitro.
Load that engine up, and run hi pitch. 11 x 7 should be ideal. - However they done like APC props. I only use the carbon props.
Stalkers are a low rmp torque engine - you must run it that way.
Agreed, teh .51 muffler is aweful, hence I run the .61 on everything small. The muffler bolts on. Those press'on's were rubbish.
( Stalker Runner since 2000 ran - 46, 51, 51se, 61, 61EX, 61LS, .66, .76, .81 )
Set them all up the same way by varied pitch and venturi.
.285 should be a good start for your .51..
I personally would ditch it in favor of the .61 more usable power and better muffler mounting. Barely a few grams more for alot more power.
However you should still be able to get good runs with proper setup.