PCI Express 3.0 card in 2.0 slot

Not a compatibility question, I'm fairly certain that wouldn't be an issue. The question is about overall performance gain. Looking at a Radeon R9 270, currently have a 5670. Here's a benchmark comparison from hwcompare.com - now I'm not a heavy gamer but I am running a mod on a particular favorite that could use a boost. Thing is, the m/b (being 2.0) can't take full advantage of the card otherwise it might be something of a no brainer. Card has excellent reviews and happens to be on sale.
So there's got to be some people here that have been in a similar situation, wondering spend the coin or not. Lets say the $ isn't an issue, just wondering about what to expect in actual gain.
Thanx.

Using a PCI-e 3.0 card in a PCI-e 2.0 slot will give less performance but not as much as one would think—maybe no more than 10% less—over what it would have done in a PCI-e slot. However, the newer PCI-e 3.0 card very well may give far better performance than an older PCI-e 2.0 card, even when running on only PCI-2.0.

Dealt with some buyers remorse but I'm pleased so far. Card is literally 3x the size of the old card. But luckily that didn't block any sata connections so no changes there, did have to reroute two m/b header pin connections no big deal and the 6-pin power adapter (included) came in handy.
No fps comparison to report but I can say I cranked up settings on one game w/heavy mod to high and it seemed fluid. Where before on med the old card was challenged.

Prior to the upgrade


Current