Spend another day testing two bikes in Dronten, and comparing them to my GT: Bülk and (again) SnoekL
same road, 4km straight stretch, ridden in both directions. Same front tires (32mm CCU's on all three bikes). Rear: Grand Prix 28-559 on my GT, CCU 32-622 on Bülk and 28-622 GP5000 on SnoekL.
test protocol: accelerate to 45 km/h, start measurement (press lap) keep steady 45km/h until approaching roundabout of Swifterbant: stop measurement (lap), Turnaround, same thing: accelerate first, at 45 start recording and stop before slowing down. I used laps so I could monitor the average speed over 4km.
My GT was ridden with normal (old-style) entry hatch.
Bülk had combi-lid, measured both short and with foam insert. SnoekL was measured open, with and without minivisor and with foam top cover (from QV, fitted quite well).
GT: 170 watts
Bülk, short lid: 165 watts
Bülk, with foam attached: 158 watts
SnoekL, open with minivisor: 153 watts
SnoekL, open without minivisor: 156 watts
SnoekL, with QV foam cover: 138 watts
note: for both open runs, the tires of the SnoekL were a little bit soft. They were inflated for the final run with foam covers. For GT and Bülk they were inflated before hand.
runs are all on Strava (for those that follow me), and laps can be viewed on Garmin Connect.
Besides the SnoekL being the most efficient, I was also surprised it's the most comfortable (for me). Also a surprise: I can store the foam cover above my knees. The SnoekL may be overly big for me, but that's the bonus of it

I'm 182cm. Length-wise I could fit a small Snoek (when using 140 cranks) but my shoulders don't fit and I like how the big SnoekL offers more room for luggage (it's a little wider, quite a bit taller and a lot longer), has more ground clearance, smaller turning circle and more room for wider tires (the wheel wells are wider than on small Snoek). I think it's fair to say it's like how the Milan GT compares to the Milan SL. If that similarity also holds for aerodynamics (which I cannot test), they will only start to differ measurably/noticeably at high speeds or real strong headwinds.