I agree that if it's just a question of moving some mass 1/2", that shouldn't measurably affect anything. I gather from others, though, that 18" wheels may be significantly heavier than 17" wheels, so that even with slightly smaller profile tires (to keep the same outer diameter) the overall weight is still greater. I don't know if that's true or not, but I was basing my comments on that assumption.
As for T-Rex, I really don't drive that section of I-25 very often, and never at rush hour. Conditions may well be very different in September, too. You might check the T-Rex web site: http://www.trexproject.com/ I'd say either try to avoid it or allocate extra time for the drive.
BTW, thanks for the CD - some great tunes, and an interesting mix.