Er, Nail, I'm not entirely sure that we'd want to know just why Question's ex-DM thought that trap was good for ECL15 - though, I must admit that it would be a good experience in how not to build a trap.
I also am somebody who's picked up ideas from these boards, I don't believe that admitting to learning something new is a bad thing, if you don't learn anything new, then what's the point?
On a sidenote, assuming an 18 as the starting ability for the save and increased at all levels, making it a 21, and it's a good save, and the relevant feat that's a +16 modifier at level 15, you could reasonably assume a few magic items giving bonuses of +2 or +3 each, that's still only a modifier of +22-25, at best, no save should be successful less than 25% of the time (for the DC40), assuming optimal conditions, because everything spent boosting that one save reduces everything else, which very few people would feel comfortable with, except maybe in certain settings, for example if the DM is known to use that particular save very often.