I have never modified the flashholes in any way. So, I have never done any comparative tests between drilled and unmodified flash holes.
I have found, however, subsonic loads with cast bullets/normal brass to be completely unproblematic. I have never found motivation to drill the flash holes, since it wouldn’t solve any problems for me. However, I shoot mostly loads with normal/heavy bullet weights. And I alternate between subsonic, and more potent loads.
If you’re loading genuine «squib» loads, it might be useful. Another strategy to avoid increasing headspace in brass for squibs, would be to shot loads every once in a while with sufficient pressure to obturate the brass. Like «10grs unique», or «16grs 2400», or something like that.