My Ek9 will eventually have the spare wheel well cut out and made flat, full weld in cage etc and there's definitely no return from that. But it's my car, if I upset somebody online for butchering an Ek9 that's their problem.
My particular bonnet isn't any lighter than the oem that came off to be honest, it's very solid in it construction. It doesn't sag or bend when it's up on the bonnet stay and doesn't wobble at speed what so ever.
I would suggest having a custom system made using larger silencers but still free flowing, a true 2.5" even 3" system can still provide the gains but be quiet enough for all race tracks.