Even on a Mac Pro, editing and especially rendering HDV is a slog. Outputting screening copies to tape or DVD from an HDV timeline takes hours, whereas it’s relatively zippy from a DV timeline.
HDV to DV conversion will take place overnight. Conversion time is roughly 40 minutes per hour of footage on a Mac Pro. Drive space is cheap and becoming ever cheaper. Using the method I describe ensures there is only one format conversion, from HDV-whatever, rather than going from HDV-DVCPro-whatever.
The only problem I forsee is one of media management; ensuring FCP can always find the HDV version of a DV file. So far, I haven’t had any problems in testing, but I’ve ben bitten by Media manager before.