I am fairly certain you CANNOT run a 28 GPM pump. I am almost sure you will have a 25 GPM valve which is one of the most expensive parts, especially if it is auto-cycle. I'm thinking, perhaps, Timberwolf did away with those a few years back though, which is why I am double guessing myself. There are a ton of other considerations. The ports on the cylinder most likely aren't big enough for more flow which will cause resistance and heat which is the main enemy of hydraulics (among others). I have a 2006 model year TW-5 that I hardly use anymore.
We have talked on other threads for a few years now. (I'm pretty sure I remember the exact thread where many members said to never start a firewood operation) I hope I was not one of them. It is very rewarding, extremely physical/brutal (whichever way you want to look at it) and has many different facets/niches/other avenues you can take, especially after years of doing it.
I've seen what 2 and even three able bodied guys can do with a splitter of that caliber in short time. I have the TW paint worn off mine to prove it. I built my first splitter and was on the cusp of building a processor when I found the one I own now. I learned a boat load about hydraulics from reading many many posts from muddstopper. He is an old-timer from NC that has gathered parts up to build his own processor over the last 5 or so years and chimes in from time to time on, what are usually very specific hydraulic questions. There is another very hydraulically savvy member, sam-tip , who built a killer splitter out of a retired golf course style fairway mower.
I've had said on here, among other places, that we try to eliminate the "next biggest Achilles heal" of our operation. In my opinion, a second or 2 quick cycle time is not worth it. That's increased wear and tear on the machine overall from several different systems (heat, faster motion, more tension etc), an increase in the chance for an injury (and they do happen), the motor and oil(s) life, changing out hoses that still have useful life (and a pump that isn't bad, I assume).
If you could swing it, and you are running a couple of guys for help, I would try to swing a kinetic splitter. I have a low dollar one I bought second hand in really nice shape for $800(?). It does need a hug every couple of months(lots of grease, tighten springs, new or tighten rollers, replace misc hardware, all low cost). We like it so much that our next investment will be a SuperSplit HD. They can split faster than a human can do any other task: load, unload etc.....just my 2 cents. I think the investment in a bigger pump would be a good fraction of what I got that used kinetic splitter for....