I'm glad you chose to experiment... And it paid off!
The Q150s are pretty well regarded iirc, I think that they will be a definite improvement over the Polks. In general, the KEF Q series does very good for itself and may indeed punch well above their respective ticket.
Keep in mind, that matching the Front 3 is the most important, and I am a fan of bringing the Surrounds into that equation (5.1 audio mixes benefit greatly from matching those channels to the Mains and Center imo). Rears are still a bit of a bastard child as program content going to them is still wildly inconsistent, though their usage has seemingly increased in more modern theatrical mixes. (Some 5.1 audio programming (DTS 5.1) does actually include a Rear Center channel rather than a Front Center, which will then be parsed to the Rear Surrounds if installed.... So even though I say it's less important to voice match all of the Bed Layer Speakers, I have in my room.
)
Regardless, those Q150s have been seeing some good sales recently, and if you are able to snag them on sale, I think it should be hard to pass on.
Just from a comparison point,
@curlyjive , please post a screenshot of your new Center "before" graph from Audyssey so we can see what, if any, changes happened in the response.
Cheers!