As both a Dali Dealer and a PSB dealer, I might be able to assist you.
Our demo room is very similar at 18 by 16 by 8 and the mentor 6 do fine, they are actually a tad bass shy compared to the Helicon's so your room size should be fine.
Personally I don't see this as being a big upgrade over the Syn 2 which are good speakers, I do think the Mentor 6 is a better speakers, I would just love to see you make a larger jump if you are going to swap speakers.