Which controller are you using for the grobeams?
The gla site is a little unclear for me if the standard controller is capable of handling 2, since they do state it has 2 channels, whereas the upgraded controller is capable of handling 4 (directly stated on the site)
As far as $'s go, they seem to be priced *fair* for the features and coverage as long as they work as advertised and you accept that it is still a new and upmarket technology. They look like one of the better options in a pendant style setup.
What is the working coverage of a single tile? It sounds like about 18"x18" each from the site, so 2 should be perfect for your 90p. a version capable of 24"x24" would be awesome for same pendant style and features.
As far as flow on the g6 goes, I personally think that if the filtration capacity is good for your desired maintenance interval, and stems in dense plantings placed in the weakest flow area of the tank will wave at you, you should be good.
I am excited to see how these perform! Your tank is helping me decide how much drool factor I really want to price in to my next project budget!