Well,
Threads bashing Pentax are quite common place here, so you're is a bit refreshing
However, the Canon D30 is one generation before K20D, if you want to be fair, compare it with the D40 or the D50. (The K20D was released in between)
As for you question, I shoo t nearly always at 800 iso (indoors and low light, no flash), there is some loss of sharpness as compared to 100 iso, but the degradation is looking like fine grain (provided you reduce chrominance noice) comparable somehow to medium format.
Common statement is that images on the K20D are clean up to 1000 / 1200 iso. Above you have to really need it and pratice a lot before getting good results.
16-50 is considered as a serious contender on pro league as long as you get a good copy. I don't own it so I can't comment.
Pentax are usually very good, about the ones that I own :
31 ldt is stellar, great colors, very high resolution, creamy bokeh...
50 / 1.4 is a nice lens but not as usefull as my 31ltd or my 70ltd
70 ldt is a great little lens, bokeh not as good as 31, and a bit slow (2.4) so I'm considering the 77ltd (much more expensive and bigger also)
I don't do studio shooting so I can't tell about your needs for this specific setup.
Regards,