That's a very good effective range for a almost stock rifle.
When I bought my Bar-10 about a year ago it would only do about 130' with about the same hit probability. (Then again I had KSC .30's..)
From my experiences Google earth is pretty accurate where I'm currently living, but it's always reassuring to check it manually with a tape.
Do you know the current fps your getting?
Did you leave the airbreak the stock full length?
May I ask how your compression is with that new x-ring? (If you don't know how, take off the stock along with the outer barrel, cock the cylinder that is in the receiver, then firmly hold a flat, high density piece of rubber on the cylinder head, then just pull the trigger, the piston should stay in place when it reaches the compression area for a good amount of time.
If it does have excellent compression; would you mind telling me the size of it, and where it can be obtained?
(The #13 o-ring I bought from HD Is having major compression problems at the moment.)
Are you planning on upgrading your rifle soon?
Good luck on your rifle.