Warranty- I can only say if Sean does the work (or Squall) if something goes wrong, you can send it back to them and they will fix it no charge to you.
As for the board itself, Sean made a thread on ScionLife for people wanting to DIY. Heres the link. Should answer the question better than words here can
ScionLife View topic - * tC : Center Tach Conversion Guide
Accuracy- its spot on. Now getting the needle right, if you DIY, thats on you.
If by lead time you mean down time. I sent my gauges to Sean on a friday, they got to him on Sunday. He was done with both the conversion and my LED swap by monday and had it in the mail to me tuesday. Bout a day for the work, mostly just shipping from Hawaii to Vegas and back.
BTW Style 6 on that link (at the bottom) is the other guy here on Oahu with this mod that Sean did a custom face for. He beat me to it cuz he did the wiring himself, while my car was at the dealership for a squeak.