My setup's the same as the above (tweeters mounted near the mirrors, crossovers and mid in the doors). As far as picking tweeter placement, my adv would be when you are putting them in (or getting them put in) experiment. Tape/or blue-tak the tweeters somewhere, listen to a bit of music and then move them around. Try on the dash, in the doors, A-pillars etc...
As far as dashmats, can't see a problem as long as it has holes in it for the tweeters ?
As far as infinity speakers, good stuff :) (assuming you mean the KAPPA50.5CS ??) . If so, on their website they are advertised at U.S. $329.95 rrp. If that's the case, then (just IMHO) that kind of money would put you easily into focal (polykevlar) and boston acoustics (rally) territory and perhaps it might be worth checking them out !! (again just IMHO :) ), But if you've auditioned the infinity's and like them that's what matters.
Also, I don't know about what speakers will fit in the ed doors, but is it possible to go larger than 5.25 ?? (not that there's anything wrong with that size (my splits are 5.25"), but typically eg. 6"/6.5" splits tend to give more midbass if that's what you prefer).
btw what will you be powering them with ?
Just my 2 cents.