Power isn't so important without balanced handling. When I look at an engine swap, I primarily look at the weight itself. Some swaps lower it, others raise it, others don't affect it, so I generally try to avoid swaps that raise the weight, regardless of any additional power. And in any case, if I end up doing the swap, I make sure to dial in the suspension and erase as much understeer as I can. But, even if a swap doesn't raise the weight, I avoid it if I think the chassis won't be able to handle the power. For example I 'm never going to put a VTEC screamer in the 1985 Panda, regardless of its effect on weight, I mean come on now. Maybe you have a different taste for swaps, but I clearly want mine to feel somehow authentic. That 's why I did the V12 swap for the XJ220: it was meant to have a V12. And I actually managed to make it handle properly, therefore the chassis must have been designed for a V12. Of course I did drop the power to roughly match the stock V6's 542bhp.
Can't remember now which races I tuned the Mangusta for, but I do remember it handled well without a wing. You might need to cut down on power and focus on stuff such as the suspension setup, tyre choice (balance between grip sufficiency and smooth reactions beyond the limit) and transmission gearing. Straight-line speed doesn't mean much if you can't carry enough of it safely through corners.