I don't think this is that big of an issue on a HD gun. If you purchase a used 870, insure that the shell stops are in good working order and fully insert the shells into the tube well past the shell stop. That HD 870 is going to sit in a corner, not bumped around in a cruiser that could cause a shell to slip past the shell stop. The home owner also is not very likely to need to reload on the run where they may not get the shells fully inserted past the shell stops.
On the older 870s without the Flexitab, the lifter can be modified with a central slot, using a drill and a Dremel tool with a cutoff wheel. If short-shucking causes a jam, the offending shot shell can be pushed back into the magazine with a key, a pen or a pocket knife. I made this modification to my tactical 870. It was easy; however always make it a point to shuck the gun vigorously to avoid this remote possibility.
John
Remington thought the potential jamming issue was enough of a problem that they took the time and spent the money to engineer a fix.
The OP says he may use his shotgun in a 3 gun If he has a jam, then what? He might be able to cycle it through with the flexitab. If not he's done. He's not going to have the time deal with the problem. Why not mitigate the potential problem and get an 870 with the Flexitab or kit to retrofit an older one?