Anyone ever had any experience using the 83-minute tapes
in LP and getting two hour of record time?

No serious user will ever touch LP mode. Tape is fragile enough
in that regard, no need to challenge it even futher. For that
reason I will personally not use the 83 minute tapes either.

The first question is why do you need it. For some event? It
probably might be better to go with a harddisk recording system
for almost unlimited recording time. I do beleive such things can
be rented as well.

As Ken says:

" In general, 80 min. tapes are not a good selection for most applications. The tape is thinner (in order to fit into the miniDV cassettes) and that can lead to some durability problems. Don't use them unless you absolutely, positively have no practical alternative. Stick with standard 60 min cassettes and always shoot in SP. "