Most jap bikes are built for riders around 75-80kgs & the spring rates are close to perfect for that weight, I ride my TTR hard on off road / dirt not to Moto x speeds & I find it pretty good.
Spring rates should be controlled by rider weight & style of riding, the damper rates set to fine tune, you must also remember the TTR is a dual sport & not a Moto x bike or an R1.
Have you considered raising the weight of the fork oil to say 7.5?
On previous bikes(serows)- you are able to put the spring under increased pressure with the addition of a spacer- have you tried that?...or take up that New Years resolution of a diet! (last option)
i used a couple of washers on top of the spring , and raised the oil level by 10mm each side and the dive has gone but still has the TTR nice compliant suspension