i've been trimming the front tinwear piece to get it to fit in my 36hp engine bay - how sealed does it need to be - i assume as sealed as possible, but it's 'heavily' modified and their are gaps.

IS this a big deal?

The better the seal is, the more exhaust heat you're going to keep out of the engine bay. Small holes shouldn't make much of a difference as long they are few. I will put my hand up and admit that my engine bay seal is perished. I am not having overheating problems although I do know that it isn't good for it. Does get a big toasty in engine bay with gap there.