One of my X's came with an aluminum rad from one of the vendors you mentioned. The seller of the car told me he bought it from them about a year ago, there is a receipt from them for it in the paperwork that matches that time frame, and its identical to that vendor's pictures/description exactly, so I have no reason to think it is not from there. Unfortunately it has two leaks at the tank seams (welds), a broken mounting stud, a bleeder vent that isn't very good, and of the two electric fans that came with it one is non-op and the other does not draw much air volume. However it does cool very well, after installing a stock fan in place of the non-op one and refilling the coolant level after it leaks out. But very difficult to get it bled with its bleeder valve.
I am not bashing any vendors nor their products, simply reporting what I have found for this particular item. And this one may not be representative of other rads from there; it could be a lemon or may have been damaged by the prior owner, or who knows? If you are considering buying an aluminum rad from one of the above mentioned vendors, and wish to know where this one came from, send me a private message and I will reply discretely.
All that being said, I am a proponent of aluminum rads for increased cooling efficiency...something these cars can use.
Anyone remember the Scirocco rad mod, before the vendors started producing aluminum replacements? Can't beat $40 for a new rad with much greater capacity than stock.