To add on to Skydork's post -The rear shock is a major weak link in just about any motorcycle. Especially when pushed near the limit. Riding a bike is about confidence and that comes from predictability and having the proper amount of traction. If the bike is squirming and sliding, there's no confidence. Keeping the rear wheel in contact with the pavement at all times is key. And that's where the rear shock comes in.
Stock shocks are usually built to low quality standards for a cheap price to the OEM. They are only as good as the OEM wants them to be at the price point they're going for. Since each rider is different, they typically don't have the adjustment needed to be dialed in for every rider buying that bike. The spring rate, ride height, damping rates, and adjustability are all very small in comparison to what would would get with an aftermarket shock. Aftermarket shocks typically offer a much wider range of adjustment, have better internals for smoother operation, and are able to be rebuilt easily. You can buy upgrade parts for them too to tailor them for specific environments/tracks/riders.
They are expensive, but the OEM shock can be modified to upgrade them to perform quite well. Not as good as aftermarket, but better than stock. You can usually respring and revalve an OEM shock for about 1/2 the cost of a quality aftermarket shock. It depends on what your goals are though and your riding ability.
If you're fast already and over-riding the rear shock and losing grip on the rear, upgrading would be the next step. If you're not that fast currently, then the shock upgrade won't be as noticeable. But upgrading it will provide better feeling. Just don't expect to drop a lot of time off your laps just by upgrading.Thanks for all the info guys!
I had Dave Moss tune the suspension once and I did notice a big difference. But I really did not spend a lot of time on it that day. I'm headed to BW on the 10th. Maybe I will have him help me again, and this time I will ask him about upgrading.