A few answers:
1. After hitting level 19 I did skip as many enemies as possible, except for nice tight groups of former humans, sergeants and captains - quite a source of ammo, I should say :). Sometimes, though, I did get carried away and cleaned the floor just for fun.
2. Gothic Boots do slow you down, but with Necroarmor's speed bonus your total time per more is 1.03 s, which is fine with me. I did not bother to remove them each time (not that there was any free slot in my inventory). And there is enough lava in deeper levels to make me want something like that (even though plasteel boots with boots protection mod and TaN(3) give the same effect).
3. Full auto on Beretta is extremely powerful indeed, but is quite a waste of ammo and forces you to reload way too often. On the other hand, single shots are fast, pack quite a punch and often push back even Barons, Revenants and Arch-Viles, which is enough for one-on-one combat. Once I found the Jackal, though, I relegated the Beretta to side-arm duty only. Still, SoB(3)+SoaG(2) give Beretta's full auto almost the same power as the plasma rifle (with much better accuracy).
4. Necroarmor is HUGE - thanks to that permanent 6 ammo, you get damaged so much less in it, even factoring in the life-sucking. I'd say that it was the absolute best find for me in the whole game... I carried Malek's Armor in backpack for quite some time in case the Necroarmor becomes problematic, but that time just never came :)