Position wise I think I’d struggle to live with ceiling speakers for stereo regardless, in wall is a different matter. There should be too much of a reason why they can compete, that said the question is really do they exist? There are some larger in wall HT speakers but less aimed at the consumer Hi-Fi market. I think there is more choice of in-wall speakers that can realistically compete with good stand mounts, the PMC Wafer series come to min which are similar to the ‘I’ series. The transmission line is just configured a different route to accommodate the shape although I’m not sure how the baffle changes things. It should be noted though PMC haven’t update the tweeter used in the current version and I think is the same as ‘+’ series.