On the electricians forum the general opinion is that premium panels, i.e. Sunpower, Sanyo (Panasonic) BenQ will add about 8% extra generation per year.
So on a standard South facing system with no shading its about 280 extra kilowatts per year.
So in financial terms, that’s about £48 extra in FIT income and £15 in electricity savings.
So if a premium panel says £2000 extra. Your Return On Investment will be an extra five years.