I say it was more a convience factor. If all they had to do was a little bit of work to get them up to their specs why not do it? They don't have to use GM engines either for the tranny and engine to match up. If a car company wanted to they could make an engine that would match up with a stock GM tranny.
"They took 5 4T65E's back to Sweden. They intentionally grenaded two of them to find the weaker links within. When I say grenaded, they ran them pretty much like Frank Gore at Shannonville but twice as hard. Sure enough, parts broke and Volvo began to take apart the three good ones to see where the parts could be improved. In all, Volvo removed 90 parts and replaced them with heavier duty parts or completely re-engineered new pieces with slightly changed configurations. They added a Geartronic function as well. The S80 was originally rated at something like 275 HP and 295 ft lbs of torque. GM complained stating that their 4T65E could only handle a reliable 280 ft lbs. Rather than stepping on any toes Volvo agreed to post the S80 at 268 and 280. True output is indeed higher and now that IPD has introduced ECU upgrades for the S80, this engine is making 330 HP and 352 ft/lbs of torque - RELIABLY and without ANY ill effects directed at the tranny. I've also been told that this is still only 80% of the transmissions reliable capacity so add another 20% to the totals and you'll be looking at a spec that the 4T65EG can handle. I hope to have a complete list of these 90 parts and engineering changes so that we may investigate compatibility with GM parts and perhaps find a way, other than Level 10, to beef our trannies for the long run."