Factory neck seals are always sized to accommodate the smallest neck that the company is likely to see for a given suit size. Either that, or they use the same small seals on everything. Consequently, for people with larger necks like you and me (16 1/2), a substantial amount of trimming is required to achieve a proper fit. For replacements, I use XL seals from OS Systems and still end up trimming them a bit.
Although I haven’t done any diving in a dry suit, I do know that the suit is pressurized to equal the external water pressure, in order to maintain loft and insulating ability in the underlayers. Consequently, there is essentially no pressure on the seals one way or the other and they should behave the same way they would in a paddling suit above water. You would actually have more external pressure on the seals of a paddling suit when submerged than in a diving suit. I’m not sure whether that would make it more likely to leak or not, considering that it’s not just the seals that get compressed, it’s the entire submerged area of the suit. It’s an interesting question…