It all is a matter of total drop. The amount of travel "Throw" that a strut has is based on ride height, geometry, and the mount of travel in the suspension. Changing ride height alters all three parameters. With excessive lowering 2.5"-3.0", you might run the risk of bottoming the strut out. That is not a good thing as it provides an uncomfortable and unsafe ride. The other aspect is a shorter life on the strut. Performance aftermaket struts are usually identical in build to the OEM strut but with different valving. I believe the TRD struts are such a strut. Other, performance struts, are designed with lower ride heights in mind.
From personal experience, I have a Hotckis Stage I TVS with 1.7"/1.9" drop and OEM struts. I had a great ride that handled itself exceptional with no bottoming out from about 7K - 41K, miles. About a 1.5K miles ago, I notice the ride beginning to get sloppy, with corner translation lagging somewhat.
To answer your questions:
1. The Springs relay the ride height, not the struts, The only exception would be adjustable coilovers where the spring perch is not fixed.
2. Aftermarket springs on OEM struts (low mileage) should be fine, as long as, the drop is not beyond 2.5". A 2.0" might prove to cause more wear than a 1.5" drop per se.
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