Tick Performance has G8 subframe connectors for $229. That is an unbelievable price for them. I already purchased and installed mine. Less than hour to install in your driveway. Great mod. Get them while you can.
Deal still works, just bought a set for myself. Didn't realize these cars even had a center tunnel brace until a few weeks ago. Previous owner must have removed it when they installed the exhaust. Picked up a scratch and dent brace from BMR and it looks perfect.
I ordered a set of the subframe connectors too. I'm not a huge fan of BMR's tunnel brace though. It might be entirely sufficient for the situation, but two bent tabs welded to the ends of a tube, isn't the greatest for fatigue life. The tabs will want to bend right at the welds. Welds are more brittle than parent material. If the tube were to continue past the bolt location, and the bent tabs were cut to go around the tube, kind of like a saddle, it would be stiffer and have a much longer fatigue life.
I might ditch those socket cap screws. If they are 12.9 grade (common for socket head screws), they are great for strength, but the material is susceptible to hydrogen embrittlement and heads popping off.
I like Spohn's brace better because it doesn't have any welds, but I think it might be too long to fit with the subframe connectors. Or maybe the Halo brace. It looks to be about the cheapest to build, if it weren't for the cosmetic touches, but is more than the BMR. There's also an aluminum one I found on eBay. Not sure about that one, not a lot of specs on it.