Joe,
Not to change intent of this thread but being new to USPSA, ever since I first shot USPSA I heard you dont want to go to slide lock. I use Dawson followers (19 in mag 1 in chamber total of 20 rounds)that wont slide lock, when I first started I never ran my gun dry, just lately I guess just having so much blasting I have run my gun dry on more than one occasion. Slide lock would have saved me alot of time e.g. knowing gun was empty instead click and what the heck.... I need to reload insert mag, rack slide. Why not use Grams followers without modifying to prevent slide lock?
I think most do it to prevent overinsertion of the mag into the gun. A 140mm mag could pass the mag catch and be forced to high in the gun resulting in damage to the extractor or ejector.