Yes I would, ... and have done so before. It would have to be a company that I know makes a good product, or based on having ridden the same model ATV that I have with the same suspension I planned to buy, or based on the recommendation of someone I trusted.
Aftermarket suspension can make a dramatic improvement in handling, and overall rider ability, both for racing and sport riding. In my younger days upgrading to an Ohlins shock on a CR 500 dirt bike improved my desert race finishes from mid pack in the novice class, to first overall in the amature class. In my not so younger years upgrading to Ohlins shocks on a DS 650 ATV improved my desert race finishes from 5th or 6th place in local desert races, to a second place finish in an AMA National desert race. Suspension makes far more improvement than a few extra horsepower from an aftermarket exhaust, air intake mods, etc. Other changes to an ATV to improve handling, or improve the riders ability to charge harder on the ATV also make great gains, and by that I am talking about steering stabilizers, aftermarket tires, better ergonomics on handlebars, throttle levers, clutch levers, etc.
So yes, you bet an improved suspension would be worth that amount.