I have an 96 LT4. The current tires on it are 255/45-17 (17X8.5 rim) on the front and 285/40-17 (17X9.5 rim) on the rear. I was thinking about geting some Kumho ECSTA tires, but they don't make the 285/40-17. What would be the downside of going down to a 275/40-17 (down from the 285's) & keeping the 255's up front? I eventually would like to go to 275's on all four corners, but don't have the money for new rims for the fronts now.

You can do it if you must , but the bigger the better, I would think an LT4 would have a traction problem even with the 285's.....
I've run all the combo's and 285/315 is the best.......ZR1's run 285/315.......you could try 275/285............but I found the road ruts are still a problem with this setup....going to 275 all four corner's gives you the "rotation" advantage....

Don't worry about it. I've run both 275 and 285 and the difference is negligible.

Okay, correct me if I'm wrong, but I thought ZR-1's ran 275's up front???

I did the EXACT same thing you are talking about PolarLT4, except I have an LT1. I'm very happy with my decision. The Kumho 275's look really good. I love how they don't stick out from the rim at all. I love the tires too. Mathematically your diameter is going to be a little bit smaller so your wheels are going to have to spin a little bit faster. It equals out to a few MPH on the speedo when you're going about 85. Not a big difference.

You are correct ZR1's come with 275 front and 315 rear.

The difference between the two sizes is 10 millimeters, that's less than 1/2 inch.(from sidewall to sidewall) I don't think the difference in the tread would be too much, at least not enough to make a difference.