Two or three shots? I haven't played it in years, but I can't recall a situation where it took me 3 shots with a sniper to kill somebody. And yes, the physics of this game were always goofy so vehicles like tanks which should have lots of torque really don't. One simple way in the game is to use the "turbo" or "fast" gear by pressing Shift+W or just E. That helps a bit, but still nowhere near real life speed uphill.
http://www.ofpec.com/ was always touted as the most complete and the best editing resource. I can't say if it's true, as I never checked it. Myself what I learned about the advanced features of the editor was from a few issues of PC Gamer (back then I didn't have internet yet). Note that to use the "advanced" features (with the scripting in the init line) of the editor you have to click the upper right corner somewhere if I remember correctly, but don't take my word on that. The simple way to put infantry into a vehicle is to put a waypoint on the leader of the group, then put the waypoint on the empty vehicle (making sure the vehicle is unlocked) with type GET IN and they will execute the operation when you load the game. If you want to start the game with them already in though, you'll have to use the init line and name the vehicle and write the scripting commands "this unitname getinvehicle name" or something like that, I can't remember very well.
Here are the amazon addons I saw on the site.
http://www.sendspace.com/filegroup/9GajEqwBcmkNDJoR11ZAhw Playing with simple addons in the editor is fine, but you can also refresh the experience with mods like the FDF and ČSLA which include whole campaigns and somewhat change the gaming experience. One reason Operation Flashpoint was such a good game was the insane number of high-quality mods and addons various people made.
I'd like to point out beside the already mentioned FDF and ČSLA; Operation Frenchpoint, Red Hammer Studios, some Russian and Ukrainian addons that for the love of me can't remember who made them (one had VDV units and various others, the other had Ukrainian army units), Earl & Suchey's marines and naval infantry off the top of my head. There's loads more I never even tried myself, so you are welcome to.