So I'm building a stealth character that is just using pistols and sniper rifle. Is there any point in investing attributte points to reflexes, or should I just invest into cool for additional damage?

Depends. If you look at the Reflex perk tree it'll tell you what the minimum Reflex score is to unlock a perk. If any seem useful then you'll probably want to boost Reflexes to unlock them (bear in mind you'll get some extra perks and bonuses just from using the weapons).

If going Stealth I wouldn't neglect Intelligence and/or Technical though. There's some incredibly useful abilities via hacking (memory wipe, reboot optics etc) you'll want to make use of, and Technical will help with the occasional locked door.