Funny, I was the one asking for advise! Happy to share what I know. Keep in mind it's from limited true experience, but quite a bit of research - hence the long response. The turbine side of the 4788 is significantly bigger let alone the compressor side. Since stuff has evolved over the years, I wanted to get some input on spool up. For a race car, it doesn't matter as much as a "street car" but I don't know how forgiving it is. Its a 7 second turbo. There are other options if 7s isn't your goal.
The turbo's listed as "GT4202, GT4508 and GT4718" and their "R" ball bearing versions are "garrett" authentic turbos. There's a GT4708 that I left off which is a 80mm version of the GT47. the "18" is their 88mm version.
GT4202 (PTE 7675 GT 42 style)- journal bearing 74mm compressor, 75mm turbine (same as PTE XX75 turbos in a GT42 footprint)
Application: strong effort stage II motor that wants to keep some streetability and move on from 3 bolt turbos
GT4508 - is an in between with a 80mm compressor and 80mm turbine. From what I know this will bolt in where a GT42 series turbo will on the exhaust side but not compressor side. If you read other forums, most think it's a waste. It's probably there for packaging or class considerations.
Application?: just skip it.
GT4718 - (was PTE 4788, now 8884) is a original garrett GT series 88mm compressor and 84mm turbine. This normally only comes in a T5/T6 exhaust housing. I confirmed that there are special T4 exhaust housing available for the Gt47 series - believe that some TSO racers are running it. I suspect it's a GT42 turbine housing machined out but have not confirmed that.
Application: max effort low 8 or 7 second TSO type car at 3400lbs.
For someone shooting for a 9.0 @ 150mph type street car, seems like a 4 bolt PTE 6870 will do the trick... Likely like a BBC type of spool up with a big cubic inch v6 but will hit the backpressure cork prior to those above. Do I know this? not really.
For garrett, the "X" version are all billet compressor wheels and should be worth net net more flow and higher boost. Question is where was the limit of each series of turbo. Seems like the GT42 was on the edge with the original cast wheel in the largest turbine A/R and the X version simply max's that. The gt47's are very popular as ultimate/ultra street turbos until they are outlawed... still popular in twin big block applications short of the pro-mod serious of turbo.
Keep in mind that PTE has their versions of the compressor wheels and at least in the 7685 and 8884 are their versions completely with quite different compressor and exhaust wheels with their "CEA" technology. If you have simply "looked" at some of the new turbine wheels lately, you can literally see the HP in them.
Last there are 7675's and 8847's (many good reviews here) which are in conventional Precision "H" cover compressor housings and smaller turbine housings. Definitely better for packaging. In theory they are both solid 8 and 7 second turbos respectively, but are just slightly more of a cork vs. the big GT housings.
I have a friend with a max effort stage II headed setup that's larger than most on cubic inches. I'm trying to convince him that a GT4788 will do him good from a back pressure perspective and let the engine breathe a little better. He has a street car that is never driven, so.....
I generally always recommened a turbo that is just big enough or just slightly big enough for your real goals to optimize or lower back pressure as much as possible. That'll also help ETs as much as possible.
Maybe Cal or someone with real world experience can chime in and help out.
