The OP specifically asks about maximal speed in an 18' Jensen in a very shallow and TWISTING river.
I don't paddle in racing hulls, or fast in any boats, so I don't really have any opinion or experience to offer. However, the answers seem to be contradictory so far: level trim, slightly bow up, slightly bow down.
The bow down advice seems to be premised on the bow "rising" at speed. I have two questions about this premise. First, suppose you can't get up to the requisite speed on a very twisty river or stream. Then what's the best trim? Cruisers will often shift weight forward to increase ease of stern skid turning, but I think this bow down trim probably slows overall velocity.
Second, in what respect does the down trimmed bow "rise" at some certain velocity? Does the bow freeboard actually increase from the gunwale line to the water line - thereby equalizing freeboard trim along the hull? Or is this rise simply the entire bow section of the hull being lifted vertically by the bow wave, with the bow freeboard remaining in the same trim down position with respect to the waterline? If it's the latter case, then a bow down (plowing) freeboard would still seem to be in a bow down plow even if the entire hull angles upward on the bow wave.
Again, I don't paddle fast enough, or tandem, to notice these effects. My unbrilliant, non-racing and simplistic view is that a properly designed hull for speed is designed to run at level trim. Otherwise, presumably, the designer would have designed in a different resting trim.
I do notice slowing in shallow water, but never thought to trim forward in my solo canoes to alleviate this. Not sure I understand what "popping" is from the descriptions so far or how it relates to trim. Can you "pop" at level or bow up trim?