Rear suspension alignment on all mk2-3 Polos seems to be a common problem.
My mk2 doesn't particularly suffer from the leaning that Alex is talking about but the rear beam is way out laterally.
Mine being a breadvan has the ususal rear tyre/arch limitations but when lowered 60mm and with 5 1/2"x13" ET38 GT steels and 165/65 tyres fitted, the offside tyre is only about 10mm at its nearest point from the (unmodified) arch return lip. The nearside has more like 20mm+!
I'm gonna remove the axle mounting brackets and slot the mounting holes so I can centralise the axle.
So, back to the leaning issue. The fuel tank sits very low and virtually central across the car (only the filler neck is on one side) so I can't see this being a problem - and this is more than counteracted by the weight of the exhaust back box anyway. Years ago, many cars had the fuel tank mounted high up against the rear wing in the boot area, and they didn't all lean over!
Yeah the engine is offset in the bay (though its not as if it were terribly so), as is the battery/pedal box/servo/steering column. This is all quite heavy stuff, but the gearbox, expansion and washer bottles, rad/fan motor and majority of the wiring loom etc. do at least go some way in balancing things up!
Remember, all the above weight is in the part of the car least effected - the heaviest part - the front. I don't see how this would/could effect the back suspension to the degree it does, particularly when people are still having problems after they have replaced the suspension?
I'm sticking with poor body alignment as the most likely problem. The afformentioned beam mounting brackets can make the car appear lop sided too, check your tyre to arch clearance each side.