My opinion, based in several years spent in erection of heavy industrial plants.
The expression "destroy the steel" is too heavy. What you're going to do at using oxyacetylene is to increase the steel grain size and its carbon content. This, however, will take place in a very narrow thickness around the hole wall, say a few tenths of a millimeter.
So, to avoid endless discussions and keep your client happy, what you can do is to grind those few tenths of a millimeter off with a grinding wheel. Now, your client should know that by using plasma you'll avoid the increase of carbon content, for there is no carbon in plasma, but not the grain size increase, which is due to heat absortion by the base metal.
Comment nº 1. Does the structural steel designer know that you're going to cut holes in the beams webs? You better make sure.
Comment nº 2. You say that you cut beams web openings in the shop at all times. The reason is that there are no naughty inspectors fooling around.
Giovanni S. Crisi
Sao Paulo - Brazil
Might want to check to see if there is, cut hole, roughness language in the contract documents also. I was happy to point that wording to a rude, condescending contractor a few years ago.