The stock shifters are mushy mostly because they have a rubber joint between the handle and the shifter base. The aftermarket shifters have stronger springs in the bases to help center the shifter in a quick 2-3 shift.
The internal stops work differently in the different models.
The T-5, T-45 and T-56 use a "finger" under the shift anvil that navigates in an "H" pattern guide under the shifter base. In the T-56 it is a bit further forward under that square plate on top. This finger is the stop and limits the travel of the shifter.
On the TR3650 there are two hard, plastic washers on the rear most shift rail. One is visible in the shifter cavity that limits travel of the rail towards the rear, and the other one is inside the tail housing that limits travel towards the front.
Yes, the forks are brittle and weak. Some of the forks have been redesigned by Tremec to be stronger. Others are available as aftermarket products.
Remember that the proper adjustment of the stop bolts is that they do not touch the shifter handle. This is writtin in the instructions packaged with the shifters. Therefore, they only offer peace of mind and psychological support... True, if the internal stop wore down, or somehow failed, the external stop would help, but the internals don't have a habit of failing...
1965 5.0 Coupe /MAF/EFI/Astro A-5 T-5/B303 Roller Cam
Mustang Manual Transmission Specialist