That engine wasn't used much I'd bet. If you look on the 3rd line, the specs are with backfire screens installed. From many conversations with WWII Allison crew chiefs, Joe Yancey has been apprised numerous times that these were rather instantly removed when the planes got to the front lines. After the pilot knew how to start an Allison, they weren't needed.
Backfire screens were needed in pilot training, but not once you had any experience to speak of. Some time back I posted a pic of one that I have, but here's a re-post.
View attachment 308717
Looking at it, you can imagine the back pressure it would cause ... and it sits right in the intake manifold to cylinder head junction, so it's right in the way of incoming air-fuel mixture. I believe a pretty decent drop in power would be anticipated ...
But, again, I appreciate the spec page. Most that I see agree, but the one earlier is not an Allison page, it is from a USAAF manual, and I hadn't seen it before. It doesn't disagree with Alison's spec, but it gives more data points, which is always welcome.
I know Joe can get about 2,850 HP or so to the prop if he builds a racing Allison for Reno using the stock supercharger and G-series rods. The extra power comes from added rpm coupled with more boost (up to 110" MAP). There might be more there with a new cam and a few other goodies, but finding out requires a bucket of money and someone who wants to actually DO something with the resulting engine. Doing it for fun isn't in the cards or the bank account.