No the 660 tank does not fit the 700R. Remember your fuel pump is in the tank on the 700 and there is no petcock. I would email or call and see if they have a tank ava for the 700R to be sure. If it's the same PN as the 660 tank then their goofed up.
It is a lot of extra weight. I put the IMS 4.3 gallon tank on my 400EX. Rode it twice with that tank and then I went back to stock. That extra 15 pounds or so of gas makes a difference. The front end didn't want to come up at all, stopping distance got longer, felt top heavy in off camber corners... I couldn't think of one reason to keep it on. I loved how it looked, and the idea that I could ride all day on one tank... BUT, unless you're riding alone...your friends are still going to have to go back and fuel up, so you're really not gaining anything by having the extra capacity. Maybe if you are a long distance desert racer....but how many of us actually do that?