I was wondering if anyone know if 17x10 +15 deep rim Work Meister S1s can fit on the rear of a 1989 GTU?

 

That depends on how you define "fit". Those will have quite a bit of poke. You'll need at least rolled fenders, preferably pulled too. You'll also need tires with a good amount of stretch.

 

​​For reference, that's a 17x9.5 +35mm offset with a 275/40/17 in the rear. You'll be sticking out another 26mm (or ~1in).

 

Sit down and do the math. Then go to your current wheels and use a ruler to see how much more it will poke out. Then the same on the inside towards the struts.

 

FC3S Pro v2.0: Mods - Rims

Food for thought.

 

^^^That link is good and provides a starting point, but the fitments are a bit conservative.

The 17x9.5 +38mm wheels on the rear of my car fit with 255/40 tires without issue. When I switched to 275/40s they just barely rubbed on the trailing arm while taking hard corners. I added a 3mm spacers bringing my effective offset to +35mm and the tires just clear now.