"So X as to Y" is a valid if somewhat rare/particular idiom. The most common occurrence is probably "If I may be so bold as to suggest . . . " Here, however, notice that the simpler "so bold THAT" wouldn't work, so we need another idiom. However, we can get away with the usage in C. It's emphasizing the EXTENT to which painters were impressed by showing what they did. If we said "so impressed that," we'd be emphasizing the RESULT--the painters were impressed, and so here's what happened. Both are valid, but they have difference shades of meaning and intent.
However,
menonrit, as others have noted above, we can't overlook the parallelism flaw in A. We need "ignore" to match "designate."