Sell Sunshine? I thought you two would only leave that boat in a pair of wooden boxes!
From what I've seen and read the L410, would slightly outperform the L400, and more than slightly outrun the L420 and L421 with similar load, sails and skill. I'd put
money on it out performing a new L42 as well.
The advantage of the L420/421 over a L410 is quite a bit more space, payload and separate
engine compartments. As you know, these may be viewed as a mixed blessing. Not sure that I'd swap a nicely refitted L410 ( eg. Sunshine) for a more expensive and unknown L400, that offers similar space and performance.
That's the trouble with upgrading Lagoons, the newer models seldom outperform the older ones. The exception being that a new L42 may sail faster than a L420, the later being an unusually heavy vessel in the designed hybrid
power version.
You don't really wish to part with thay lovely L410 after all your TLC, do you?