I've been thinking more about this one. Decrements are a blunt instrument, because they try to cover a range of situations in one simple figure. So they can only be approximate, and have to cover a worst-case situation. In the exact situation you had the other day, the availability of that short (10%) bonus of clearway would probably improve the contaminated RTOW very slightly, if full and accurate calculations were available for with-versus-without. But because of the limitations of the decrement system, it's possible that you might end up with a slightly lower RTOW, because of the higher decrement you have described.
PS: You described the runway as simply wet. You may notice that I have talked of the "contaminated" case. Perhaps I should explain that under UK performance rules – unless they've changed recently – there is normally no weight decrement for a wet runway, provided there is no measurable depth of water. What we normally do is to reduce the V1 by a variable figure – usually about 10kts – and accept a lower screen height at the end of the clearway (15ft). We may also assume the use of one thrust reverser in the wet runway case, but I'm not sure and it's getting late...