How do you define "slow" in this scenario? The posted scan speed for the M525 is up to 42 ppm for B/W and up to 32 ppm on color. Ofcourse your results will depend on the scanning settings you choose too.
If all of your M525's are performing poorly then perhaps there is an incompatibilty with that model and your current network configuration. A quick test would be to grab a m525 and set it up on a isolated test network. Something simple like a router, printer, computer and access to a email server or service. Test out the scanning features to see if you can scan to email or scan to folder and recreate the same results. If the printer performs the same on a test network then you know there is a limitation that does not meet your needs. If the printer performs better on the test network then start isolating what could be different and find the bottleneck.
HP Support is not going to be very helpful on this subject. I dont see how a XEROX support tech would be any different if they sent one to your site. This is either going to be a physical scanning limitation of this printer model or a network configuration issue. In the end you gotta do what is best for your company, no hard feelings. I'd assume there is already a reason why you dont have an entire fleet of XEROX machines to begin with, they arent always all they are cracked up to be either.
Considering the amount of money you spend with HP I'd advise looking into setting up an HP Account rep. Account reps are normally assigned to big accounts like yours and can get you acccess to NDA's and additional support. For example, if you work with an HP Account rep you can set it up so that you can evaluate HP Printer models before you buy whole fleets of them at a time. This gives you some time to properly evaluate if the printer will perform as needed before you get too invested.