gearing a sled down REMOVES load from the engine, allowing it to use more clutch weight to be at a specific rpm, so you answered your own question. By having the sled geared down your going to need more weight to keep at a specific rpm, the load is less on the engine and thus requires more weight. I would gear the sled up(larger top sprocket), you will load the engine more and it will be faster, the srx likes to be overloaded and tractor its way up in the rpm range, they are consistantly faster this way. The engine rpms will respond by dropping and allowing you to run the weight you already have. Gearing up also keeps the belt in a better ratio in the primary clutch, a belt radius that is close to the same for the front and rear clutch is where youll have a cool efficent running clutch and the sled will be fast.
Worn secondary cam buttons can cause high rpms as the clutch doesnt smoothly slide and has extra friction on the ramps of the helix, you can also blow a belt with those.
You can place a nut on the opposite side of the bolt head, some spiders are closer then others so double check all clearances. Usually the thread will stick beyond the other side of the weight with the long bolt and 1 or 2 washers, allowing you to place a thin nut on it, but beware that the nut does not interfere with the roller or the inside of the spider. blue loktite is a must on this hardware.