Yes I did actually. No vendor was prepared to sell me the genuine diffuser without the $3000 exhaust system, so I got the eBay one instead. I havn't mounted it yet, but it looks quite good for the price. Finish is gloss black (my option) and it is quite good, I'd rate the finish an 8.5 or 9 out of 10. The exhaust shop seems to think it should go on just fine when they looked at it. I am booked in to get the exhaust made and fitted in 2 weeks time.
For the time being, here are some photos of the diffuser itself. sorry they're indoor iPhone ones. Note the "HARTGE" stamped into the diffuser with the Hartge part number too - seems this was modelled off a genuine diffuser...