In my experience, you will eventually run into an issue, even if it appears to initially work. We've gotten bitten with odd issues months after an OS conversion (like printer settings on a specific report or an update that completely cripples FileMaker). There is absolutely no support for FM8 on 10.10.
Filemaker 8 is coming up on it's 10th birthday. 10 years ago the mac OS was 10.4 for a little frame of reference.
If you're going to upgrade your hardware and OS, why not give the same respect to filemaker and plan an upgrade?
Agree with Mike, especially since your into the post FM6 generation of product. Conversion is not usually a big issue.