I think you know the answer to this! But yes, a kayaker will make a better dragon boater. It's all about the cross training and their ability to develop a wider, complimentary group of muscles. As well as that all important water sense.
Also, spending a lot of time in the kayak, surfski or outrigger you will develop a very keen sense of what exactly makes the boat go faster. To be able to knock out 40-80km training session in the outrigger you need to keep your focus throughout the entire training session, and this equates to keeping focus in a dragon boat race.
Training a lot in the canoe as a team is a really great selection and trainign tool. There's no where to hide in a 1 man boat! But in a 20 crew DB, you can guarantee that there are people not pulling their weight. So the individuals start working harder in the 1 man craft, pushing themselves and their team mates on, which then equates to more power in the DB.
For example the Aussies train a lot in the outriggers and they did a fantastic job in the Festival, getting a silver in the mixed. SPC also train a lot in the 6 man and 1 man outriggers and have been succesful in the DB both domestic and overseas.
However, I've always felt the DB short races 200/300m do need a special skill set that only DB can really nail. Canoeing will help build up the anorobic/aerobic/V02 threshold very well, but a short race your technique and synchronisity needs to be spot on. With only 80-100 strokes to play with, it doesn't leave much room for error, and it's my opinion that solid DB training is the way forward.
I've heard a rumour that SDBA are buying OC1's to compliment the NT training programme, so maybe it is the future?