Well... The difference is a bit more then 15cc.
In il135 you have 30x14 2blade. On il150 you have 30x16 2blade. And this with a little more rpm on ground. So, 15cc are not able to do this. But the weight is fact.
The model will decide who is faster. P51 is faster then spitfire.
You see the difference in climbing up.
Did i post the video of the 2p51 with il135 and my spit with il150?