You could start your own BBS with this topic. The Westside K27 compounds win the big prize. I think the early slide valve versions got a correct boiler but the boiler rides too high on the frame (a problem shared with several models - WSM C25's, Lambert's C&S 2-6-0), Thankfully they got it straight for the modern slide valve #460 - the one I really wanted then (for some reason).
The old Key C18 and C19 were real bad. I never could get either to run right especially the C18. The frame is milled out of tram. I tried requartering the drivers (they were out of square) and it ran WORSE. To further complicate matters, the prototypes were documented at the same time when 318 and 346 had swapped tenders. So the models came out that way. That was fine if you bought both models (maybe that was the plan!)
One of the last thing WSM did was the post-wreck RGS 455. They went all the work to make the new "oversize" cab and tender only to make them the same size as the "stock" K27 pieces. Very disapointing.
Then there is the mid-1960's PFM K27 that was an interesting comination of 453's power reverse and doghouse and 454/456's inboard piston valves.