As the system is in series the FPC may become a radiator when the ETC starts working at a higher temp, unless your store is large and lowers the flow temp into the FPC sufficiently.
I am not sure if it is efficiency that varies really, more to do with losses (which can be calculated as efficiencies I suppose depending on what you measure).
The question is why would you want to do this if you have 10m^2 to play with.

All down to the physics, looks and cost.

As I understand it, yes, it's all about the simultaneous losses rising to equal (or even exceed) solar input, and so:
1) heat pipe collectors maintain efficiency (somewhat) at lower outside air/radiant scenery/sky temperatures;
2) flat plate collectors (FPC) are less efficient at lower ditto because more succeptible to simultaneous re-radiation loss to cold scenery/sky;
3) evacuated tube (non heat pipe) collectors (ETC) are just as succeptible as FPCs to simultaneous re-radiation loss, but a bit less succeptible to simultaneous conduction loss to cold outside air.

1)'s efficiency is relatively unaffected by thermofluid temp
2) and 3's) efficiency rises sharply as thermofluid temp is lowered - but how low can that go and still be useful?

I have a hunch that 2) and 3) would be more efficient, in mid winter, that 1), provided thermofluid temp were lowered to say 22C. Our theoretical calcs show efficiency of 60% in mid winter, with suitably optimised (but simple) FPC, at that thermofluid temp. At 60%, with big (but simple) collector, insolation is adequate to space-heat a near-PH building by solar alone, right thro Dec/Jan (depending on horizon obstructions).

Interesting, thanks Steamy.