I'll be interested in the responses to this. I just machine quilted some pads using low-loft poly batting and a variety of old sheets and cases. The easiest were the poly/cotton with lower thread count, although they were more slippery and needed pinning. I had 2 high thread count cottons. One was easy and one (i think 500 or 600) was just awful. I'm not sure one could conclude much from that about all sheets. Except that the high thread count seem a lot heavier than quilting cotton.
hugs,
charlotte