I dunno, but my HomePNA ran fine for gaming at 1 Mbps. The 10 Mbps one would probably drop down to 1, so I wonder if manually setting it 1 would improve things.
I did have a problem once though with a crappy line splitter. (I had voice, DSL, and HomePNA all on the same line, with a 3-way line splitter.) Paid $5 for a new splitter and everything was fine. Also, one could try those line filters and see if that helps since these technologies can sometimes interfere with each other, if you're running them on the same line. Third, old lines sometimes are inherently bad, but it seems yours work fine.
Jack's explanation doesn't really apply here though, since gaming does not require high bandwidth. All it requires is low pings reliably, and HomePNA does this quite fine if set up correctly. HomePNA didn't seem to add any significant latency for me vs. Ethernet. Maybe a couple of ms at most, which is inconsequential. Remember that 1 Mbps HomePNA will max out the bandwidth of most DSL accounts anyway. With my 1 Mbps HomePNA card, I was getting consistently over 700 Kbps on my 1 Mbps DSL acct., and sometimes hitting over 900 Kbps.