There comes a time - maybe after a decade of paddling? - when any additional gains in VO2 or strength endurance will be minimal / even measurable, or when we're simply trying to slow the regression of what we once gained. In that light, if we see improved performances, it will be from non-physiologic things like improved or more efficient technique, better posture in the ski, improved ability to use the obliques and legs, etc; and from using our heads - knowing how to catch the runs more effectively, relax in the ski, take better lines.
If you're on the rising part of the curve and have lots of potential improvement to come, then you can do more with speed and looking to try to improve it as the season(s) progresses. If you've been training for awhile, like I have, then looking to continually improve speed will be frustrating because of diminishing or negative returns.
This is the approach that I take. The training, i.e. HRM vs speed centers around first being fit with a good base, and then how to maintain freshness without overtraining. Getting "base fit" is simply lots of miles with good technique and that is arguably best measured by time in the boat rather rather than with an HRM or with speed.
To get race ready, know what your marathon race pace HR is, then do your long intervals at that HR (long intervals meaning about 8 min+). If you are doing shorter intervals, there is too much lag in HR response time, so therefore those are best done looking at speed and perceived effort.
To maintain freshness, I limit the long max effort intervals, i.e. in the 2-6 min range when in-season because these seem to peak me and then trash me.
And, of course, your results may vary. Best to formulate your own training plan from a combination of your own past experience with paddling and other sports, what the academics say, and from what the top guys are doing.
Erik