You're doing a POC on WatchGuard?
Well, you won't get the figures here since typically no responsible SysAdm or NetworkAdm will gloat about what fw is deployed.

What you can do is to get the WatchGuard Account Manager to connect you with other WatchGuard customers, than you may ask them etc and go through reviews etc.

From my experience, WatchGuard is ok for SME's, but not for a full scale Enterprise especially a telco or ISPs.