There are two types of speedo pickups, see the how to page. RE-pete, your boat probably has the old style. The new ones can be used but the MDDC needs to be reprogrammed. Accuski can do it and turn it around in a couple of days. The new style are supposedly not as susceptible to clogging.
I used JB Weld epoxy to fix my expansion tube, after blowing out the hoses and cleaning both venturis, I have not had a failed speedo in 2 mos.
The GPS idea is a good one. Now we just need to tie it into Accuski!