I want to build a character that is similar to Leon "The Professional" in the gun skills but prefers being sneaky with a knife but as an adept/mage (which ever works better) with abilities similar to Karl (whose skills are => Leon's) in Ted Dekker's "Saint".

Karl can focus and alter his body tempurature, but he can, with effort, alter the tempurature around him. With finding out he can do that he tries to apply what hes doing while meditating (the tempurature thing) to his sniping. He is able to control the bullet in flight out to 1200 yards (the length of his target range). He even alters his bullets path to enter at an angle where no real damage is done (muscles are torn, but no bones or organs are hit). He also goes into a deep meditation where he is pretty much useless for anything but this focus and manages to survive an electric chair.

Some of this sounds like adept stuff, but stuff like altering a bullets path seems more like a manipulation spell. So is this just specific instances to use abstract stats like a really high longarms skill? or is it entering the house-ruled power/spell area?

Eh, I'd argue it falls more in line with Improved Ability: Rifles.

Afterall what exactly about the ability makes your shot more accurate? It isnt really quantified, as it's magic, not actual -skill- (beyond your control of said magic) that improves your shot. So arguably, that improvement could well be explained as at least some degree of control over the bullet flightpath.

As for the minimized bullet damage thing.. eeehh.. that would really have to be some sort of HRed additional power I'd say. It still sorta flies since there's allways constant references in SR rule books about 'undocumented' magic/metamagic techniques from all walks of awakened life. Afterall they dont explain for example, what ever the hell Winternight did to the nukes. Just that they 'magically enhanced/altered them. There's some mention of it being like turning them into a weapon focus nuke. But nowhere does it -really- say what the effects of this magical alteration actualy -did-, or at least, was supposed to do.

As to affecting the temperature -around- you. Ehh... thats really the realm of magic. So then you'd be looking at some sort of temperature altering spell. Since the rest fits so well with an adept, I'd go for one of those Adept Mages. Dump the excess points into perhaps specifically some temperature controling spell. Altering body temperature.. eh, bit more iffy. Arguably this would fall under the adept ability of temperature adaptation. Though it doenst really acutally raise/lower your body temperature far as I understand.