Speed holes on Phils car:

Rear diffuser on 5G's:


This one is a photoshop

Im not really up for the holes in the bumper but what does everyone think the diffuser?
Is it a worth while mod for my, what is essentially a track only, car? The car is sitting low and has a deep front lip and sides. Would a big bit of perspex sheet do the same job? Something like this?

Ideas please.