You're on the right track with regard to engine size; a .46 is certainly no faster than Bronze, and a .61 will not likely put you into Gold.

While almost everyone on Gold is running a YS, I've seen YS-powered WM P-51s turn Silver class times. In other words, the course you fly makes a big difference.

The regular World Models P-51s are a little lighter and less expensive than the glass-fused Dago Red, if that makes any difference.