An HP2553 is just a somewhat arbitrary bundle of an HP2551-C, a WS80, a WH32B, and a WH40, while an HP2551 is a bundle of an HP2551-C, a WH32B, and a WH69E. The distinction between what is an add-on device and what is mandatory is just a marketing distinction that thankfully results in a bundle discount. In these bundles the only item containing a pressure transducer is the WH32B. One could probably throw together an HP2551-C, a WS80, a WH40, and a WH45, and the console would probably work just fine. Minor console firmware tweeks at best would be required. One just wouldn't have a pressure sensor in the mix.
When it comes to the GW1000, as it is the only "WiFi gateway device" that directly incorporates sensors (one of which is a pressure transducer), yes, a pressure sensor within a WH45 used with a GW1000 would double up on pressure transducers. On the other hand, a WH45 used with an HP2553 doubles up on both indoor temperature and humidity sensors. A case of choosing your poison I suppose. There isn't a lot of cost concern in mashing up a HP2553 with a GW1000 wherein indoor sensors for temperature, humidity, and pressure are duplicated, as this happens frequently it seems.
On the other hand, reducing the sensors in the WH45 to just PM2.5, PM10.0, and CO would avoid duplicating sensors with current products. Not obvious what sensor combination they're driving toward long term though.