i did the same thing. i ran the whim update cable to the right side speaker pod. it isn't like they are releasing new firmware updates for the whim at a rapid fire pace anyways. i think they've done one in the past year?
depends on what you want out of the system. if you want to be able to have full functionality, using siri for navigation, text, commands, etc. then yeah, get the whim. if you are okay with limited functionality, ie. CarPlay, then just get your system flashed for the whim and mic disabled. that'll give you CarPlay and you can use your headsets to communicate rider to rider, but you won't be able to use voice commands or send/receive phone calls through the boom.
one correction, you can use non-hd branded senas with the whim, you just would have mono sound vs stereo.
look at discount sources instead of your local dealer. i got a 2 pack of sena 30k headsets for $450, and $250 for the whim. bought one whim locally as i had a store credit and chrome cash points so out of pocket for that one was $50.
i will say that there is a definite learning curve with this stuff. at least for me it was.