I've mixed Shimano Ultegra, D-A, and XTR cogs to build custom 9-speed cassettes with good results. The XTR that I used was a 12-34. 12, 14, 16, [18-20] [23-26] [30-34] The larger cogs are pinned together on common carriers and cannot be used separately. From this cassette and other parts I built a 13-26, my favorite 9-speed cassette. At one time I had my wife on a 14-28 9-speed.
I cannot detect any shifting degradation mixing Shimano cogs, but I always try to keep the ratios as close as possible and still get the job done.
I definitely would not try to run a crankset with anything bigger than a 50 with a 34 inner ring. That would be like shifting a car from 2nd gear to 6th gear, the engine would probably die. I hate big ratio jumps and corresponding cadence jumps. I run a 53-39 crankset but would rather have a 52-39 or 50-36.

Al