If you are able to safely seat the bullet out to a longer OAL - that is, if the chamber has enough leade or freebore to allow the longer length without interfering with the rifling (Jamming into it) - THEN the increase in length, will increase the usable case capacity - but by how much?

That depends.

You would have to weigh the different case capacities to find out, weight-wise, just how much of an improvement you made through long seating.

Seems to me, an increase of 4gr. of capacity, is equal to approximately 50-100fps when both were loaded to the same pressure.

Now, simply seating the bullet out as far as possible and using the same load as before, will drop the pressure and velocity as you have technically increased the powder chamber- this is only if the bullet is not NOW jammed into the throat. If it is, pressure will be increased, possibly to dangerous levels - not a working load any longer. If the bullet can be seated out and still be clear of the lands (throat) by .020" or so, then you may load it to normal pressure and improve the performance, probably by a small amount that may or may not be noticeable anywhere other than on a chronograph and even then, it might show no change.

Lee Factory Crimp Dies allow crimping bullets over shank portions as this system uses a collet which impresses the crimp, not a tightening of the seating die's interior with can cause case buckling.

I do not believe there are any hard and fast rules regards expected improvements. One experiments and takes his chances - might or might not.

Much also depends on the powder used, how efficient it is in THAT case with THAT bullet and THAT primer & as well, how efficient THAT cartridge is in the first place - how close it is to being too big to allow small changes in powder capacity to make any noticable difference in performance.

Playing over seating depths, is almost like playing with wildcats. We IMPROVE a case, straight it's sides and sharpen it's shoulder and raise it's capacity by about 10%-15% well over 10gr. increase and sometimes it only develops 100fps more speed - or perhaps only gives us the same speed as before, but having to use more powder to do it such as my .220 Swift IMP - a full 8gr. increase in capacity, no increase in speed - oh well.