I was wondering if there was a big difference between the Infinity Kappa Perfect 6.1 and the Kappa Perfect 5.1 speakers? The installer said he wouldn't install a 6.5" component set in my car but he could do a 5.25" because I have 6x8 openings in my car. So i was wondering if I should just let him do the Kappa Perfects 5.1's or should I try to find someone who will do the 6.1's?
It sounds like you desperately want those 6.1's. Any installation is possible with two things - imagination and money. What i'm saying here is that if you are looking at fitting into the stock locations you are limiting your options, but with some customisation you can fit whatever speaker you like.
In terms of difference, there is always a difference in terms of their performance across certain frequency bands. The 6.1's 6.5" woofers will perform better in the lower midbass region with a greater low end extension. the 5.1's 5.25" woofers however perform better in the mid region which is the region that we are more sensitive to.
Just remeber that it is all down to your personal preference as to which is going to reproduce the music you like.