So far as I can recall, my adapter is just like Tony's and I don't think there will be any obstructions to good flow caused by that centre bolt. In depth investigation will have to wait until the next time I take the filter off. I will be sure to report back if I find anything interesting.
For what its worth, last time I changed the filter I also removed the oil cooler a previous owner had fitted as I think the oil was running too cold in my TR7 and causing sludge in the sump. The stuff in the bottom of the sump was the texture of thick honey and about 1cm deep. See picture.
I had always used good oil and changed it frequently so that was a surprise. I thought about using the oil thermostat I have in stock but decided it wasn't necessary on a road car and took the whole lot off. The cooler pipes came off a modified transfer cover with a couple of threaded outlets. Its all sitting on the shelf if anybody needs one.