Disclosure - I'm a former Sunpower employee. I didn't pay street prices for any of my panels.
In theory Sunpower only allows sales of their panels installed, and they won't do boat
installs. Any panels sold without installation
are serious grey market but they are sometimes available. Sunpower does sell cells to other vendors (Solbian for example) who then put the cells into modules.
I have 4x230W Panasonic HIT panels and 2x230W Sunpower panels. All are about 6 years old. All have nearly identical specs - 19% efficiency, 42.x Vmp, 5.4x Imp (they differ in the final decimal). We have them installed to try and make sure they get 'equal' shading (as much as we can).
Have done a lot of datalogging over time (amongst other things I did performance guarantee work/validation for Sunpower). Short answer, the HIT panels produce about 6% less total energy over the long-haul compared to the Sunpower panels. About 1/3 of the difference is evident in the temperature coefficient, which is slightly higher with the Panasonics, so they perform a little less well as temperature rises. The rest is mostly due to slightly lower performance in lower light conditions (before 10AM and after 3PM or so).
We charge LA batteries
, so we don't tend to use all of the available solar
in the afternoon, therefore the Sunpower panels might gain a bit more in afternoon but my data won't support that because we've moved off the maximum power point.
In the end, the absolute best you'll see might be a 10% performance bump from Sunpower panels over other high-efficiency panels. Boat systems tend to be very space limited and that 10% may justify a significant price bump. But if you can find just a little bit more space then the HITs and their other high-efficiency brethren probably make a much more cost-effective solution.