Twin pipes on the old 488 doesn't increase the top rated horsepower by more than about 5-8. However, the power range and response is better. Probably not worth the money and tuning issues unless you get them for cheap and you have the time.
As for a later y pipe, I would have to say not likely, as my 1997 Trail RMK has the same one my old 91 Indy 500 had (I doubt the liquid vs fan matters there as they are both Fuji 488's).
The IFS bars are different if I recall. As for the kit, you MIGHT be able to find one on eBay or something.
The Xtra-10 in the rear DOES make that much of a difference.
Hope this helps.

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1997 Polaris Indy 700 RMK - Twin SLP pipes, EGT gauges -- "Fukengruven"
1997 Polaris Indy Trail RMK 488 - Stock -- "Yard Sale"
1980 Polaris TX-L 340 - Stock