An air leak is never good theoretically but very small air leaks will not appreciably affect main jet size or peak power but may affect idle mixture and a lean condition under closed throttle deceleration. 

Has the engine experienced a broken piston skirt or broken thrust washer? Has the engine had a connecting rod bearing fail where the big end of the con-rod rubbed the engine case?  If the engine has experienced any one of these 3 types of failures, you cases may have damage causing the cases not to seal together at their mating surfaces

Have you run the engine yet?  If so, does it seem to smoke excessively?  If so, does the exhaust smoke smell like some other type of oil other than your two stroke oil you are putting in the fuel?  Is the transmission oil level going down?     

1.  Does the engine rpm return to an idle slowly with the ring ding dings after blipping the throttle?

2.  If it does return to an idle very slowly, is the throttle slide completely closing?  If the slide is not completely closing adjust the throttle stop screw so it will close.

3.  If the throttle is completely closed and the carburetor is jetted properly and the engine still wants to idle excessively high, the air leak is large enough to cause a  problem.

The Power Valve design for the LT 250s and LT 500s makes the engine very difficult to accurately pressure check.  If the engine shows some leakage during a pressure check, the power valve may be responsible. 

If all of the critical machined surfaces were checked for flatness and scratches and the proper sealers were used on these surfaces, pressure checking should not be necessary.

I have been finding a lot of leaking cylinder base gaskets and reed gaskets on cylinders that have had the ears welded the last year or so.  These two gasket surfaces were not machined after welding on the cylinder and should have been machined!!!!!