I have not seen a definitive description as to what can be done in the way
of modifications to either "Special -LSA" or "Experimental -LSA" vs.
"Experimental - Amateur Built". It seems that S-LSA cannot be modified by
the owner/operator without the manufacturer's approval - Is this the
equivalent of obtaining an "STC" (but issued by the manufacturer instead of
the FAA)?

What about "Experimental - LSA"? Can the owner/operator make significant
modifications without the approval of the manufacturer - as is the case with
the "Experimental - Amateur Built" and still be "legal" according to the
type certificate issued?

Here's why I'm asking - I would like to develop and test several
avionics/autopilot/flight-control products and I will need a test-bed
aircraft in order to do this. The amateur-built category doesn't seem to
apply because this isn't just for personal education, etc. - there's a clear
commercial intent and application.

What should I be looking for? A certificated aircraft with some
waivers/permits, etc. or an experimental?

Thanks in advance for any input.