You are getting at a problem I have with the Central Supply Catalog, especially as a player-facing resource. Its got some pluses scattered through it that make it too easy for one or two players to be playing a different game than the GM and other players if they're starting from the core rulebook. I dislike that at a meta level, I'd rather have everything on the table from the beginning, and without having to powergame myself to keep up. It might be time to take a look at whether you really want to keep those mechanical options in the game as straight stacking pluses (I really think that +2 should have been +1 on a 2d6 scale, or they could have done something with Boon instead but the numbers were down before Boon was in the game). And if you don't want to remove those, at least take a look at what else is available and come to some decisions about what you're using.
But, you are leaving out some numbers if the player is getting straight to a 2 Can't Miss. Opponents can dodge (and should, rather than stand there and take it), they can dive for cover, they can take cover to begin with, they can wear armor, they can target the most dangerous opponent preferentially to make him dodge and take the penalty to his next attack (and if you take the skill scale seriously, while he doesn't personally have 4-Famous, he certainly has it effectively), they can have smart guns of their own... There's options if you're playing the full game.
And all that said, Traveller combat has traditionally been deadly, over quickly, and more about ambushes, positioning and superior firepower than most other rpgs, so in a broad sense things may be working as intended. As a GM you may have to vary your encounters. More individual combatants, drones or trained animals, a really big alien creature that isn't going down to one or two hits, an ambush, a sniper in an overwatch position, cheap disposable muscle sent in with stunners in a civilized area so if they win they get bailed out and if he wins he gets mindwiped or prison planeted... I'm sure there's more too.