I'm using the HKS mid pipe with my T78 kit. If I had to do it all over again, I would have cut my RMM style DP from BPU and had a flange welded on to mate with the GReddy DP.
The HKS mid pipe is a bit smaller in diameter than the GReddy pipe or the RMM style DPs. Also, I found the HKS mid pipe to hang down a bit too low on my car. I ended up having the GReddy downpipe cut about 3/4" and a flex section welded into the HKS mid pipe for a better fit and more ground clearance.
While I was at it, I also had an extra hanger welded onto the Tanabe Racing Medallion to make that fit better as well...
Hope this is helpful,