The Mac Pro runs Yosemite just great. I'm going to put an SSD in it, as the system's worthwhile again now.
Quad 2.6 GHz Xeons and 13 gigs of RAM is enough hardware to last 8 years and still feel fairly current, apparently!
But this all depends on the GPU. I upgraded it with a sibling of the card that's in the modern Mac Pro (and similar to the PS 4 and Xbox, which makes me smile as an old ATI fan back in the day). Yosemite doesn't exactly tax a card, but it must support it.
Don't think laggy with an unsupported card. Think unusable. All sorts of things don't render at all, so you can't see what's going on. When I first got the Mac Pro up on Yosemite, I shrugged and thought "oh well", then I found I couldn't bloody shutdown as things weren't drawing and I couldn't tell what was going on. Had to pull the power cable eventually. After some cursing, I popped in the Radeon, expecting trouble and perhaps even needing to install Windows so I could firmware flash it. You can imagine my surprise when everything was transformed without an ounce of effort!
So far the only stumble I've found is Facetime. Won't sign in on that machine for some reason. But iCloud works fine otherwise, including Safari's stored passwords in iCloud Keychain: which is often a no-go on Hackintoshes. This one is an authentic Mac, so such things are much easier.
Do your research and see if anyone's got Yosemite working on the specific GPU you mentioned in the MacBook Pro. If it works well, you could pick up a bargain as most people would consider those Macs obsolete now. Always nice to wring a little extra out of them; while plunking away on a keyboard decades older still!