Shouldn't be. With a 17x8" you should be able to run about 5.5" backspace before you get into the outer tie rod. The guys running mustang rims are usually using 17x8's with 5.72" backspace and a small spacer to get to around 5.5-5.6" backspace.
But, apparently not all lower ball joints are created equal. I've heard of as much as 6" backspace working on the front on 17x9" rims, and as little as 5.4" with a 17x8" rim hitting the tie rod. Only thing I can figure is that the steering arm on the ball joints aren't all at the same angle, might vary a bit by manufacturer. The design/shape of the lip on the rim might make a difference too if things are close.
Regardless, 5.25" bs on a 17x8" should be fine in the front.
