It depends on which game you are talking about. In BG1, no. In BG2, having a thief's Detect Illusion at 110 means they are in detect mode constantly. Also, you must take into account things that reduce a thief's skills. In this respect, yes there is absolutely a point in over scoring... to a point. What I mean by that is that there is no lock in the game that will ever require 100, let alone above. Traps, on the other hand are detected quicker, the higher the score.

As for your second question, Hide In Shadows *never* becomes a 100% chance: there is *always* a chance of failure. It's just less as the score rises.

One other thing: In Baldur's Gate 2, your Hide in Shadows and Move Silently scores are averaged. This means that having one of the skills over 100 may be useful if the other is less.

Anyway, a better question to ask is, at what point is the chance of hiding as high as it will get?

Incidentally, a mage/thief is likely better than a pure thief thanks to all the spells that can replace some thief skills, and it happens that Baldur's Gate 2 has no less than 3 of them to choose from. (Of course, 2 of them are dual class characters who can't gain thief levels.)

Frequency, meaning that the Hide in Shadows triggers more often, on average.
Move Silently becomes more apparent when approaching hostile creatures.

I checked the wiki (in retrospect, I probably should have done that first) and for every skill besides hide in shadows/move silently and pick pockets, it specifically states that raising it above 100 has no effect, while pick pockets is the only one that specifically states that raising it above 100 DOES have an effect. Not sure how much truth there is to that or if the wiki is outdated.
http://baldursgate.wikia.com/wiki/Thief

Baldur's Gate Wiki is absolutely riddled with copy/paste inaccuracies and misleading information. It is not a place to treat as a BG bible. Taking thieves, for example, the wiki continually states that there is no benefit for thieving skills to go above 100, yet completely ignores the fact that things like debuffs and equipment *reduce* a thief's skills, making those extra points suddenly seem oh so important. A piece of advice: never blindly believe any wiki, since they can be written by anybody.