For what it's worth -
I ride my 520exc in very tight stuff - it might sound funny, but my own back yard is -really- tight, 80% first gear, 20% second. It took a while to get used to the wheelspin and somewhat control it, but... I did, and so will you. I have not ridden a 400 by comparison, but I think anybody could adapt to the 520. In my opinion, the power is not 'viscious', just builds very linear. I must admit, though, that I rarely get past 1/2 throttle.
I love my 520, not because it's the fastest thing out there (let's face it - the problem is all me, not the bike), but because it can be so darn fun when you do get crazy with the throttle and eat big long ruts when you slide through corners, etc. Waaaaay funner than my 1100 streetbike.