The strongest motors I have seen for the GTV 6 are the big black sealed Ducellier motors. They are a real hoss compared to the cheap smaller motors used on the Spiders. Much better torque, and a heavier case. Once you lube the bearings they seem to run fine. I drilled a small hole in the back end of the casing for lube access to the rear bushing, then oil soaked the front bushing with the motor vertical in my vise for a couple of days.
As for the gear drive, I have disassembled several of these for both Spiders and GTV 6, and have not yet found one to be dried out, with stiff grease. Every one has had soft lithium appearing grease packed into the casing, lots of it. You can drill a small access hole next to the worm and worm gear, to be able to spray lithium grease into that space and onto the gear teeth. In fact, if you look carefully and study how they're built, you can drill that lube hole with the gear drive mounted in the door of the GTV 6. No need to remove it and fight the cables, etc.
I did a post on this recently, with photos as I recall. Besides the obvious electrical circuit, relay and switches, the other items that affect window speed are alignment of the guide tracks, condition of the felt channels in the guide tracks, the friction of the belt weatherstrip on the glass, lube of the cables and pulleys, and cleanliness of the glass pane itself. I'll look for that post link.