Most companies have people who are considered top performers who, under different circumstances, would be considered extremely weak salespeople. While these salespeople do appear to be weak when we assess them, it's much more difficult for management to recognize how weak they are when they lead the company in revenue. Notice how I didn't say "sales"? That's right, they happen to manage more revenue than anyone else, but that has less to do with selling than it does account management...
So how would your "top performers" do if:
You took away their existing accounts?
They had to overcome strong resistance to your brand?
They had to compete against 5 competitors for each account/deal?
You put them into a new territory and they had to build it from scratch?
Their customer contacts all retired or quit?
They had to make cold calls?
Your company had higher prices, a lesser brand, or a new, unproven technology?
They had fewer resources on which to rely?
You raised their quota by 30%?
They didn't have established relationships with their customers?
They were only allowed to close business with new customers?
In my experience, if I had to choose between a very successful account manager type who did not have to overcome any of the previously mentioned challenges, and someone who was struggling to perform but had to overcome most or all of the same challenges, my money would be on salesperson #2!