Temperature isn't a factor, even overcast isn't that much of a factor, especially for residential installations.
Thermal is more affected by overcast due to the light being more diffused but still doable, southern Europe will be good for thermal solar, all of Europe will be good for PV, and central and northern Europe is very good for hydro and wind (anything on the golf stream is excellent for wind)
There is very little need to import electricity from across the med, the loss in transmission alone not to mention the instability in the region makes this entire deal pretty silly.
If nothing else there is no reason to replace one type of mono-economy that keeps oppressive regimes in power and forces the west to balance the region on the head of a pin more often than not through coercion with another one.
The Moroccan plant is in Ouarzazate, which has an average annual irradiance of ~2,200 kWh/sq. meter.[1] Southern Spain is the sunniest part of Europe and the best locations there are closer to 1,800 kWh/sq. meter.[2] That's over 20% more solar energy in Morocco.
The other bit that's not really accurate is about Transmission losses. It's about 400 miles from Ouarzazate to Gilbatrar. Modern HVDC lines have losses of ~3.5% over 1,000km, so call it 3% for this project.[3] That's extremely reasonable for an energy project.
[1] - https://upload.wikimedia.org/wikipedia/commons/6/6d/SolarGIS...
[2] - http://www.mappery.com/maps/Solar-Radiation-Map-of-Spain.png
[3] - https://en.wikipedia.org/wiki/High-voltage_direct_current#Ad...
20% isn't really an issue especially for thermal.
1. HVDC is not a strict requirement for submarine power lines. AC works just fine, even if less efficiently. Efficiency losses are generally offset by lower capital costs. Remember that Gibraltar strait is only 9 miles wide. In fact, there already exists a 16 mile-long 400kV AC interconnect between Morocco and Spain now.
2. Even if HVDC makes more economic sense for transmission, it won't impose any sort of "electric grid upgrade" requirement on Europe or Morocco. The HVDC line would interface to the existing grid via a HVDC converter; these are built at GW scale. There's already plenty of HVDC submarine power lines within Europe now. (Britain-Netherlands, Norway-Denmark, etc).
You may argue that new infrastructure will have to be built to distribute the added capacity, but that's a different argument.