I know that the CRJ900 has a very high Landing Distance Required for an aircraft of its size but I'm surprised if the Colas BBA surface on SEN's runway is often declared WET. The BBA surface course is normally laid un-grooved due to the fact that it retains good friction characteristics during rain. Runway 05L/23R has this surface as does that at IOM.
If that is not the case then surely grooving should be carried out to ensure that the runway very rarely has to be declared WET. After all, SEN's runway length is one of its disadvantages so it would be most unwise not to ensure that such limitations are minimised.