As the tire diameter increases, the brakes lose mechanical advantage.
As the tire weight increases, there is more rotational mass which also add stress to the brake system.

Are 245/75/17s going to ruin your brakes? NO
Would 38x15.50 tires cause concerns? YES

The waiver you signed is probably a CYA document that the shop instituted for all oversized tires to defer any liability. Unfortunately, there are idiots out there that would buy huge tires and hold the shop responsible for brake or trans issues and claim negligence.

... so don't sweat it.

 

Boy, you learn something new everyday. I have never heard of a tire store saying this! rangehat:

 

It only takes one idiot to change company policy.

An example was when the shop I worked at was sent a memo from corporate that we were not to install snow tires on one axle of a vehicle. Regardless of what the customer requests we could only install 4 at a time, even carry-ins.

Story was a shop installed snow tires on the front of a vehicle only (fwd) because the customer demanded it even against the verbal advice of the shop. That vehicle was involved in an accident where the rear end slid out. A law suit was filed against the shop which cost them A LOT of money because they either lost or would have lost and had to settle. Moral is, you can't rely on the customer taking responsibility for their purchases anymore. In this example, waivers weren't even permitted because they wouldn't have held up in that court case either.

 

Thanks for the info, the tires are installed just in time, we had asnoww storm here and the tires work great in the snow and looks good with the lift. I just put 1000KM's this weekend on snow pack roads with graet traction and no brake failure as i expected. Just wanted to make sure that i was aware of any issues in case it had happen to any commanders out there.
Thanks again!

 

You wont have any issues with your brakes. That shop you brought your XK to is clueless.