650 is a bit underpowered for long distance riding, it's capable for sure, but not ideal, I make that statement with 135,000 miles on a SV650
I just had an idea, my tank will get replaced with a new one, I will be able to take the old one and modify it, maybe increase capacity to 8gal or so
A lot of guys who endurance race have done that. It's not too difficult to weld in some metal and make the tank bigger, making it look decent at the same time is a little harder.
Search around in the endurance racing section of the WERA BBS, you'll find a number of threads dealing with it.
'07 DL650, '01 DR-Z400S, '99 SV650 (race bike), '80 GS1000S, '85 RZ350, '08 Ducati 848