As soon as my front sprocket arrives I will be installling a 520 set up as well. I went with the 14 tooth front 36 tooth rear and RK chain. I also was able to pic up a QC carrier (Afam) pretty cheap. Total on the whole change is less then $200.00
Two pounds, maybe? Not enough to get all excited about, that's for sure.
Historically, racers would always switch to 520 for the weight savings and increased availability of 520 chains and gearing. We would also run cheap, non-O-ring chains for less rolling resistance and change them often. Today, that's not the case. We can buy 525 sprockets in any config we want for equal prices to the 520 stuff.
So why switch? Beats me. Some engineer type will come on and tell us about reciprocating weight and how the net effect is so much more than the two pounds static...maybe. But your bike better be pretty trick - and you better be one slim and in shape racer - to need the difference.
Having said all that, I will be switching my 749 streetbike to 520 when the time comes. Why? The cat on ebay selling RK chains and Vortek sprockets as a 520 kit for $130! That $130 speaks way louder to me than two pounds or "racer" cred!
BTW, if I may offer a tip: Run a 15 tooth countershaft and a larger rear sprocket to achive your gearing goals if at all possible. The smaller 14 tooth countershaft will increase chain wear, and more importantly the larger sprockets will keep the chain run off the swingarm and free up your rear suspension action. Trust me, this is a set-up on all Duc superbikes.