Just read Klaus's review on this lens on Photozone.de. Thought it might be of interest for those looking for the Pentax version of this lens if indeed it does come. It was tested on a similar sized sensor (canon 50D) so these figures would only be slightly less for the Pentax (assuming another copy scored near the same).
Tamron AF 18-270mm f/3.5-6.3 Di II LD [IF] Asph. VC macro - Review / Test Report
Overall I am not really impressed. It doesn't seem to offer any advantage over the current 18-250 in fact seems to offer less edge performance overall, with added distortions of the usual nature. VC obviously isn't a concern for Pentax users so the question is whether or not the extra 20mm at the telephoto end was worth it for this design?
I assume Pentax is coming out with this version eventually but that is only based on some of the forum posts I have seen here. Based on it's optical performance I'd stick with the 18-250 for those who have it, it isn't worth the added barrel distortion and decreased edge performance just so a lens can be called a 15x zoom.
Just thought that might be of interest here. I already have my own 18-250 variant and I like it quite a bit. I would not upgrade to the 18-270 based on these figures.
Carl