Hi Scott...
I'm not a "video" expert, but I work for a lot of people who are - here's what I've heard:
1) Editing HD on any standard system; is s l o www - There's a great deal more data to process, so everything takes from two to ten times longer.
2) FCP is seldom happy when mixing formats - it's best to match your project's properties to your source material.
3) HD files on the web are huge - If you have much of an audience for an HD project more than a couple of minutes in length, it would be wise to plan a budget for extra bandwidth, or figure on SD for the web.
-Therefore, it would appear wise convert everything to SD first, then edit that way. There are methods available to edit and output in SD, then replace the SD input footage with HD after you've completed your edit, thereby giving you the advantages of both - the speed of SD editing and the quality of HD.
Good luck on this one.
Travis
VoiceOver Guy and Entertainment Technology Enthusiast
[
www.VOTalent.com]