USB 3.0 can burst up to 5 Gbps using 2 full duplex lines, according to the great Wiki. The two 2.5GHz signals will have to have to be treated as RF with important frequency components up to 10x their switch frequency, or 25 GHz. Maybe some of this cost covers a VNA? The device inputs will have to have a broadband match up to 25 GHz, or at least have a match characteristic simple enough for which to compensate. Seems wonderfully difficult to me! Wish there was more info available, but the project was only opened on Kickstarter a week ago.
I have the DangerousPrototypes $45 Open Logic Sniffer, but it only samples at 16x70MHz (which is fantastic, by the way), not 2.5 GHz. (I wonder, though, with hardware buffers, 16x70MHz could be made into 2x560MHz.)