Ok first I searched for about 30 min and found nothing concrete.

I'm thinking about running 245/40/17 on the stock rims and I've searched and have seen that this setup will work. There is one post in particular by "warrtalon" saying that the gearing will be off, but he doesn't go into any details. My question is how bad will the gearing be off and is this a major issue? I only street drive my car with the occasional trip to the track. (once last year, planning on maybe 3 or 4 this year)

 

Tirecalc will tell you how the change in tire size will affect your effective final drive ratio:
http://www.miata.net/garage/tirecalc.html

 

The change in gearing is the same difference as a brand new tread tire vs one that is all worn out. So, for a 245/45, just imagine wrapping your stock tire with a layer of new tread. It may matter to drag racers that care about the fractions of a second but for daily driving you will not even notice a difference.