General system construction guideline - modifiers beyond 1/4 the die range are degenerate to the system. So for d20, modifiers beyond +5 degenerate the system. They create a skew of target numbers where those without the modifiers cannot hit the target number, and target numbers they can hit become meaningless to the character with the modifier.
Consider target number 11. Hitting this number is a 50% chance with an unmodified d20. With a +5 modifier, it climbs to a 75% chance. You are proposing a system where characters can start with +10 modifiers once level one skills are modded in. For such a character, 1 is the only roll that misses, and the midpoint of their skill roll is 20, which is again at the fringe of what the unskilled characters can do.
I've seen this in Pathfinder, and by extension it plagues 3.5. Once characters are at 15th level they're clocking around +25 on their favorite skills. Normal target numbers no longer have any meaning to them. But they still need to be called for because on any given skill there's someone in the party that cannot perform the action. Rather than be a guide to what a character is good at, skills have, in my game at least, become a straight jacket to what characters can and cannot do, at least when I'm trying to challenge the skill monkeys on the check. Whenever I do that, I insure those without the skill flat out have no shot at success.
I've been working on a system that allows player progression without this form of degeneration, where target numbers can be left static - DC 18 is a moderate check under this system regardless of whether a 1st or 20th level character is rolling. Does the 20th level character have a better shot? Yes, but he can still possibly fail though its highly unlikely. The system is largely untested still and I'm going about it at a snail's pace.