if you want to go fast with a rmk gearing is the only way to go my wifes swithback is geared high for trail riding and i can't keep up with my rmk if you have a 2" track it will be slower than a 1.25 weights will only make it shift faster but most clutches shift out at 8200 rpm
Just kind of an over view of clutching and what does what.
Primary Weights alone will not drastically improve your top speed, they control rpm (more weight = less rpm) the primary spring controls engagement
The weight itself
-Heel weight (near the mounting pin) affects the low rpm and engagement, and is useful at low speeds
-Toe weight affects the end of your shift, more toe weight is better for higher speeds
A softer secondary spring will give you more upshift, and will increase your top speed slightly, although attention must be paid to the clutch temperature because a decrease in secondary spring pressure is a decrease in side pressure on the belt, allowing for a greater possibility of belt slippage. Simply feeling both clutches will let you know if the belt is slipping on one clutch or the other, obviously if one clutch is warmer then the other, there is some slippage on the warmer clutch. With an increased upshift (secondary opening faster) you will also have a decreased backshift (secondary closes slower) which can cause the feeling of "freewheeling" down a hill.
In my opinion, changing the gearing in the chaincase (increasing the drive sprocket diameter, and/or decreasing the driven sprocket diameter) would be the best way to achieve the top speed you desire. Then adjust the primary clutch weight to achieve proper max rpm, and the primary spring to achieve proper engagement rpm (less spring pressure = lower rpm engagement). A greater engine load will be experienced to power the higher track speed. A change in the secondary helix angle may also come into play and can help combat a slow backshift caused by a soft secondary spring (higher angle = slower upshift & faster backshift) and opposite for a lower angle.
Remember driven divided by drive equals your gear ratio (ex: 13 tooth driver and a 21 tooth driven = 21/13=1.615 or 1.615:1 ratio)
As far as actual parts to recomend for use I cannot advise. Good quality adjustable primary weights should be avalible at most all rec dealers. Recreation Supply is a good website to checkout and would give a guy an idea of the parts that are avalible. There are so many variables when it comes to clutching, there is no real cut and dried answer, i'm sure some may agree with me and some may not...this however is just my opinion and what I would do if I was doing as you want to.