I emailed Invidia's European distributor to see if they could explain the difference between the two exhausts (the older Gemini and the newer Q300tl) and this is their reply:
"In regards to your questions, it is only the rear muffler that differs , the Q300 is a little more quiet then the Gemini , that said we only carry the Gemini in stock as that outsells the Q300 10 to 1."
The reply doesn't surprise me, as that's exactly what it looks like when you compare pictures of the two systems. So I suppose you pays yer money and you takes yer choice. Therefore I've ordered the Gemini (found a place in Scotland that delivers them for £992 - They import them from America. I'll let you know what they're like as a company when the part arrives - http://www.funkypower.com/index.php/).