From what I am understanding {+2} to a dice roll is a big deal. If a +2 is a big deal, is there a cap on the value that a GM should allow?
That depends on what 'it' is.
There's nothing wrong with an automatic success in something that is nigh-on impossible to fail.
The rules say you should make someone make a task check:
• when the characters are in danger.
• when the task is especially difficult or hazardous.
• when the characters are under the pressure of time.
• when success or failure is especially important or interesting.
If the player in question is trying to fire some sort of bullet-delivered homing beacon at a car, then if he's got a laser sight (+1), the car is large (+1), he's a trained marksman (+2), in good health with a high DEX (+1), and he's got plenty of time to aim (+1), then
he is not going to miss.
This isn't a problem. Especially since making someone make a check for that (and failing) can derail the adventure.
Piloting checks are the same. Assuming a decent DEX (+1), piloting skill (+1), aerofins (+2) and a 'proper' starport (+2), you literally cannot screw up. But you shouldn't be asking them to make a landing roll every single time they arrive at a planet, anyway - only if they're being chased, or there's a storm, or the ship's damaged, or something like that. Otherwise, it's not interesting and it just leads to every thirtieth ship trying to land crashing, which is clearly nonsensical.
The two reservation I'd make are:
First:
Remember that skill 2/3 is professional. Good professional, and more than enough for most players. Skill 4 means world-reknowned level - the sort of competence reserved for snipers, noted professors, Olympic athletes and thousand-dollar-an-hour retainer lawyers.
It's not impossible for a traveller player to get that but it should always be a major focus of the character's history and personality, and other people will know about it*.
Second:
Remember that there's a limit to your ability to multitask. Getting a +1 from a gunsight is fine, but a normal character can't really benefit from expert software in an intelligent weapon, ballistic tracking lenses, and a gunsight together; they're all variations on a 'smart gunsight' and you can only look down one sight at once.
Don't allow people multiple stacked bonuses unless you can understand how someone can benefit from them all simultanously (for example, in the latter case, if the player is in something like battle dress with a tactical computer that can collate all this data and present it properly, fine).
* Including NPCs, not just the players. If you're Gun Combat (Slug Rifles)/4
someone will know who you are, even if your background is something along the lines of
"I joined the Imperial Marine Corps, then was transferred to a special ops unit where NOTHING HAPPENED before retiring and joining the party."