IMHO
Place you need to start, either opt for the 150 front track spring or even better jump right to the XCR shock and spring combo. My initial problem was never ending bottom of the front track shock along with heavy steering and darting. No matter how much I adjusted, could improve it but never got out the heavy steering or darting. Next mod, XCR shock n spring combo, first ride was wow what a difference in ride but steering still far too heavy and yes it darted pretty good. Read all the forums I could find and tried n tried different sets up with limited improvemend, also tried several sets of skis (including shimming) with some improvement but not what I was looking for. Finally towards end of last season I was at the point of moving well away from what I'd already tried, figured I was at the point of giving up on it and moving onto a different sled for this year.
XCR FTS with XCR spring 1-1/4" preload (not positive but think high is on 4 and low around 2 or 3, very easy to dial in).
Front shocks / stock springs 2 -3/4 (clicker on 4)
Rear track shock / stock spring 2 - 1/2
Went away for last two days of riding logging a little over 1000kms in conditions ranging from frozen fresh groomed to loose junked out corners in the afternoon and some medium rough sections. Skis are SLP-SLT (have tried 4" with anti dart spoons / shaper bars / 8" SLP round bars as well) with their 6" round bar carbides, track is studded 3 per bar, rider around 230lb. Sled now steers well on the light side, yes even on freshly groomed but frozen concrete surface all the while providing stellar turn in no matter how loose the trails got in the junked out corners. Ride wise, by far the best it's ever been, in the air and landing over a hill, hard to know you even left the ground, eats up the junk like it never has and most important of all, it does not dart, period. Now a 1 hand 70+ mph down a rutted out railway sled, far and away better than it's ever been. Only noticed one slight downside, on a roller type bump, especially a few in a row where it was junked out going into lunch, back end did feel too soft and yes did bottom a time or two unless I stood up. Tried to dial more clicker into the shock which did help there but it hurt some of the feel good in the steering as it did not allow the rear of the sled to settle nearly as much. What I'm trying this year, bought the matching XCR rear track shock, will be installing it along with the stock Pro S spring and setting to previous preload. Idea being, have low / high speed I can likely better dial in how it works in the those rollers, feel strongly it's going to the key here.
One thing I can tell you, this combo has taken it from a sled I really liked but hated at the same time to one I can't imagine replacing anytime soon, yea it's that good.
Given where I am now, feel pretty strongly the stock front track shock plus 150lb spring cranked up to where I've got mine would make most peeps very happy, for sure a huge improvement over the sacks out way too early stock dual rate version. Time will tell and I've got two friends trying that exact combo this year (neither ridden yet).