This exact same subject was brought up on AK forty seven dot net.
A member was wondering if there were any real differences between the SA M-7 and the converted to high capacity SLR-101. The thread was pretty positive on the SLR conversion, up until a fellow poster brought up that the receiver needs to be milled "AFTER" it is already heat treated and assembled. He also stated the following:
When a part is heat-treated, the heat treating only penetrates to a certain depth. The outer layer of metal is hardened by heating, which introduces carbon atoms into the steel. Beneath the heat-treated "skin" the metal remains soft. This is okay, because it allows the soft inner "pulp" to give a little during stress, and the part is not brittle.
Now, add re-machining after heat treating. Machining as deeply as is required to enlarge a mag well undoubtedly penetrates the hardened "skin" and into the "pulp". Re-heating the receiver could increase the chances of brittleness unless the affected spot is the only part re-heat treated. This is possible, but very tricky with a complicated part like a receiver.
Not re-heat treating the receiver at all also leaves something to be desired because you're talking about some of that soft "pulp" being exposed to wear from insertion and removal of the magazines repeatedly, thus resulting in a receiver that wears out prematurely." (in that area)
End of quote
After we discussed this for a while, we pretty much left the thread believing that the areas that get machined are probably not so critical as to worry about the possible exposed soft areas, but still, it convinced some that they would rather have a receiver that was not modified after being heat treated and assembled. The thread raised a tiny bit of doubt about the whole mag well conversion thing on the SLR-101, enough to convince the original poster to go the SA M-7 route.
Realistically, this possible minute exposure of soft steel is probably a minor or even a non issue altogether, but again, it somehow had us look at the conversion in a different light. The possible exposed soft steel was not something most of us thought of.
Even though it may not be an important issue at all, I figured it would be a good idea to let you know what this discussion brought to our attention.