I think the 13 x 4 W would be ideal for the irvine with the thottle pipe..... the 13x4 combined with a Thottle pipe revs wayyy to much as it is.
Tried the 12.25x3.75....and i thought it was horrible! revved like parking warden's pen top and the noise was terrible!Good vertical and superb throttling though....

It wont punch out of the hover with this set-up though....might be worth trying a Just Engines purple pipe.
Substantial power increase. My mate gets a MASSIVE 11,500 rpm out of his irvine 53 swinging an APC 13x6 on 5% nitro club fuel!!!!