I own a 07 Phazer Mtn Lite that I bought for my wife from a buddy of mine last year. The first thing he did for it was replace the rear springs. One ride was all it took for him to pull the trigger on the purchase. At the time there was only 6 sets of heavier springs in the entire US (or so we were told). However, like I said, there was no question in his mind that they needed to be heavier so he bought a pair. $360 form Yamaha or something like that.
Anyway, you should be able to track down a set because the stock springs for the different models of Phazers are different rates.
Mtn Lite - 17Nm
GT - 21Nm
FX - 25Nm
Venture Lite - 30 Nm (although these may not fit)
All the above are in the parts fiche online.
We put the 25Nm springs on ours and it worked out great. He also made some anti-bottoming stops (which you can buy from Yamaha for around $70) and they help too. Gets the load onto the springs earlier, sorta-kinda-youknowwhatImean.
JL