Danny - what would be the cost of swapping out the GRT1s with the GRT2 tweeters that I hear are so much better?
It depends.
If you have already soldered on the GR-T1's or used them then they are yours.
If you have not used them but would like to upgrade them then send them in and exchange them. Just pay the difference in price, $10. for the pair, plus the shipping.
Would the crossover design with bb's kit work with the new tweeter.
Yep, they're drop in replacement with no network changes needed.
Could someone still order the Sonicap upgrade?
Sure.