If you're pulling a boat uphill at 75 mph the throttle would have to be wide open, or close to that. Personally I don't see a need to push it that hard. It doesn't make any difference to me if I get to the top of the hill 30 seconds sooner. Also, there's a lot of weight there. The Tahoe weighs 6000 lbs, your boat may weigh 4000-6000 lbs, then add people and gear. If something happens at 75+ mph (blowout, deer crossing the road, stupid drivers) it won't be pretty. Fuel consumption is also an issue. Just because you can pull the boat that fast doesn't mean you should.
I know there are a lot of people who routinely tow at 75-80 mph who "never had a problem". To each their own.