Explain "optimization"? You certainly don't want to reduce fan performance (CFM and SP) no matter what size pipe, so that eliminates any need for a VFD to slow down the blower, right. You effectively limit CFM by choice of pipe diam., right? The design of the blower/impeller affects how it performs (vac impellers are drastically different than DC material handling impellers). Since you are not changing the impeller other than to possibly speed it up and since that machine's primary role is as a DC not a vac, you will not see much of any improvement in CFM or SP without changing duct size. So, again why a VFD.