I don't think your gonna get into trouble if you download and install this mod - sell 15,000 copies on ebay as you own work and you might have a bit of trouble, upload it to RD and admin might tell you off.
As for installing it on your pc and having a go thats your choice and your decision, bear in mind though it may technically be breaking some copyright law or other, even if you do own NFS Shift, so then its a moral choice, given the extreme unlikelyhood (is that a real word?) of you actually getting in trouble for it.
The real deal breaker for me is the car id's clashing thing - from what I have seen it shares at least one id with the MMG F1 mod and given the choice I'd rather have that installed and running well on my pc than this - although if it were 'legal' and would work (properly) alongside the F1 mod i'd probabaly be asking admin if we could have a Racing Club Event, because those screenies look great and the author has produced some fantastic work from what I have seen.