track
The 154 and 162 indeed have different pitch drivers where the 144,151, 159, and 166 have the 2.52 pitch. That's why I mentioned changing drivers. The stock XLT only has enough room for a 1 inch track with the 9 tooth driver. Polaris made a 1.5 kit but it had 8 tooth drivers. If you go longer than that then drop and roll is th only option. On the wedge chassis, the drop is need for the paddle clearance the roll as well otherwise the paddles will hit the crossover cooler when they come around off the drivers. Flatter approach angle is and additional benefit. That makes them climb up on the snow better too.
The XLT chain case will only accept a 40 or 41 tooth bottom gear. Mated with a 18 tooth top gear will get you 2.22 or 2.27 ratio. I would not go to any smaller top gear. Those ratios will be fine for either the 144 or the 151.
8 Tooth drivers go down another 8% I believe so final ratio could be in the 2.4 range. If you go smaller drivers, I would use a 19 top gear, it will wear better.
The 600 twin does put up more HP than the triple so I wouldn't go any longer.