So, for the OP, his choices are:
* Use 50A wire, normal 40A breaker and risk often nuisance trips
* Use "100% rated" 40A breaker and reduce the chance of nuisance trips. If the inverters backfed current is spec'ed to be maximum at 40A, then the chance of nuisance trip is close to nil. But I'm afraid that with 40A breaker, the inspectors would use J.Wiles articles and dictate 32A backfeed as limit on the PV/inverter.
* Use 50A breaker and replace the main 200A breaker down to 180A breaker (for inspectors that use "the code man" articles as their bible).
* Use a higher rated service panel
