I was in the same spot as you, i recently wanted to make a hi capa build which wasn't a speedsoft esque gun. And the only options I really had were guarder.
I had to do cut off a small bit of BBU (which guarder state you have to do) to get the stock blowback unit to fit and ever so slightly file the screw hole that secures the blowback unit to the slide. But it wasn't anything strenuous.
I managed to fit it onto my stock frame no problem, paired with a 5KU outer barrel that doesn't tilt. Works like a dream.
The criticism from knuckles is warranted. However the current market trend for hi capa slides are more suited to speed soft builds.
In short, you get what you pay for.