Steven,
I had asked JBL this question many years ago when purchasing my first pair of 725's. I was thinking at the time that if I may need narrower coverage patterns for larger venues, or when running pairs. What I found in practice is that this has not been the case.
What I was told by JBL is that the pattern can be narrowed with no ill effects, however, the pattern can only be narrowed by another 15 degrees down from the factory 75 degrees with the proper horn.
Take a look at the VP series and you'll note there are two options for the horns in every type box. The drivers for the VP series are virtually identical to the SRX series and in the VP series all other components are the same. So, based on what I know and have been told by JBL I would say that if the mechanical properties are the same, this, bolt patters, mounting holes, flange type, depth, etc., and the only difference is the opening there should be no issues.
Now, the real point for a narrower pattern would be audible distance, how far away and how many DB, or splay. I have had problems with neither and can project over 100db to better than 75-100 feet outdoors with these boxes. Inside with small to medium sized venues these boxes can be anything from overkill to just the right size. After that I'm running the wrong box. Crowds of 1000 are handled well with the right amps. See the point??
Personally I would leave the 725 as is knowing any increase in performance is going to be marginal at best, or look to another type box if that is your need.