In order to have sneaking reduce the enemy vision angle, you'd have to have different colored zones (ex: Sneaking 5 is okay in this area of the cone, but Sneaking 2 isn't). That's a huge UI change and not a viable suggestion, for development costs alone.
If 30% per rank isn't enough, it could be increased. Now that I think about it, the primary purpose of early points in Sneaking should be unlocking Sneaking-based Talents (such as reducing the cost or giving you full movement speed while sneaking), with the latter points finally making sneaking cost-effective as a damage multiplier in combat.
