Welcome to the high road, Friar Whently. The stock rubber grips are superior for recoil reduction. The altamont boot grips are just about as hard as metal and leave the backstrap open to boot. There dual purpose is a)aesthetics and b)ease of concealment and draw. I would say they are slightly thinner than the stock banana grips, but wider than crimson trace 405's, and likewise wider than the current factory boot grips on other j frames. I can get a good purchase on them if I tuck the pinky under the boot.
In my limited experience shooting the 640 pro, the Speer gold dot 135 grain .38 + P had lowest recoil and best accuracy and the Corbon DPX 110 grain .38 + P was also easy on the hands and quite accurate. Both loads pack a punch in my model 442. The Buffalo Bore 158 grain LSWCHP .38 + P and the Corbon DPX 125 grain .357 magnum had about the same recoil. I found both to be accurate provided I did my job and pulled the trigger back in one smooth but firm stroke. I could get both to point of aim on a 5/5/5/5 drill (five rounds in five seconds at five yards in a five inch radius). I've found in my own shooting with the 442 that flinching, an expectation of recoil, or a low grip can cause it to shoot low. If I get a good purchase high on the backstrap and focus to eliminate expectation of recoil and flinching accuracy is greatly improved. I recall that I was pleased with the accuracy of the .357 DPX and it was not more unpleasant to shoot than the 158 grain buffalo bore load + P. Verdict is out to which of those two I will carry and dependent on more testing. At the end of the range session I had a blister sized chunk of skin ripped off the inner surface of my thumb but no bleeding. If you have arthritis or carpal tunnel or are recoil sensitive, stick with the +P loadings. That said I find it can be accurate with .357 albeit the DPX load is a moderately loaded round compared to other factory offerings.
Corbon indicated to me that testing of the 125 grain DPX round out of a 2" barrel got them 1085 ft/s velocity with a penetration of 14" and ending diameter of .70" in 10% ballistic gelatin. That is excellent performance even at the snub level (testing out of a 4" barrel puts that round to about 1300 ft/s.
I'll let y'all know which one I intend to carry although in either case I expect the DPX to remain in the speedloaders/moonclips to avoid lead in the pockets. All said and done I might just carry the DPX + P in speedloaders to serve as potential reloads for the 640 and the 442. The 442 is currently loaded with 135 grain + P gold dots with the DPX as backups and as I indicated earlier i'll decide to go with the BB + P or DPX .357 based on further testing. I did decide to let S&W take it back on their dime just for peace of mind. Its possible the ftf was due to some combination of moonclip and primer placement however its nothing a good factory checkup, perhaps a beefier firing pin, and further testing can't iron out. Now all this thread needs is more pics of J frames and I'll leave that up to you
In the meantime i'll be woods carrying the Gp100 which can outperform the 640 any day but compromises in weight, comfort, and concealability.