I haven't got an XS-650 or XS-750 but I did order one of the first boats that were made. Mine's an XS-600 with a 150 Opti. Pettal who posts on here as an XS-700 with 200 Opti (see link below!!). I'm sure he'll be able to advise you on the bigger boat.
Mine has been flawless since I've had it. The build quality is second to none and is a very solid boat. Everything is bolted (no self-tappers here!!), the consoles are glassed into the deck also. There is some nice attention to detail throughout the RIB also.
As regards their seaworthiness, mine has been to the Fastnet and back in Force 6/7 conditions
with no trouble. Thats no mean feat for a 6m boat!! The hull is 23 degree deadrise at the stern and is a warped V so will give a good turn of perfomance/fuel consumption without compromising on its rough weather ability. Also the hull is a high lift design giving it good load carrying capacity (this also gives better consumption). Trust me from seeing the capabilities of the 600, the 650/750 is definitely an all yr round boat. It's also a VERY dry RIB, you will be very surprised at how often you will find yourself leaving the wetgear in the locker.
Performance?? Mine will just about do 50kts and I think Pettal's does it easily!!
I've put together some links below for some more reading. If you've any questions don'e hesitate!!
Pettal & his XS-700 on the Solent
New XS-600 / Merc Opti 150