No. VarDiff will in time will optimize your difficulty for your miners to maximize your submitted shares and minimize stales.
I'm afraid the reason has nothing to do with stales. (And where there's said that submitted shares are accepted shares?)
In fact, VarDiff tries to
minimize ;-) the number of shares submitted, trying to conserve bandwidth and server load for fast miners.
If this is the case why offer lower difficulty shares at all for slower miners? Yes correction I meant accepted not submitted.