At first I thought this was good idea. But after playing, yeah, this is stupid! Just drop the negative stat off the mod, and leave it at that. This really felt penalizing at Level 7 and below, when you don't have many mods to begin with.
How might the negative stat attachments alter weapons when optimized for one style? Does it help or hinder build diversity?
+20% for stat A with -50% for stat B could possibly be extrapolated to +100% for stat A, -50% for stats B, C, D, E, and F... Leading to a weapon built specifically for one thing.
How might that level of specificity and choice impact later-stage builds?