Most panels seem capable of exceeding their nominal rating, which, after all, is during standard test conditions.
If we have a cool, breezy day (to keep the panels cool) and periodic spells of strong sunshine (the sun is now at its strongest) then it would not be surprising to exceed full power.
My own "cheap Chinese cr@p" panels <Kinve 250W Mono facing SE on 40 degree slope> have been touching 270W per panel in such spells of strong sun since early April.
Comparing to nearby systems (same postcode district) I see no major difference between panel types, except, perhaps, sometimes Sanyo panels slightly outperforming (but nowhere near enough to justify the massively higher cost of Sanyo).
Looking at the stats for my less-than-ideal aspect (between East and South-East-facing) 15x 250W Kinve compared to others ("+" means my panels outperforming, "-" means my panels underperforming):
20x 190W unknown make: +3%
14x 240W Sanyo: +11%
16x 245W Sanyo: -1%
14x 235W Sanyo: -9%
20x 200W Suntech: -4%
14x 250W Siliken: -6%
Average performance for my panels: -1% (and that's comparing against three premium Sanyo systems).
One thing that is really striking, though, is that with this constant patchy cloud, it's very hit-and-miss from one day to the next depending on the luck of the draw of where and when the gaps in the cloud appear.
An array only about a mile from me crushed me 12:7 on one day in May (nearly twice my generation) but on one day in June I crushed them 19:10 (nearly twice their generation).
Another array about ten miles away had been beating me by a few percent every day for weeks on end, but in the last week I have often doubled their output and their previous spectacular performance has now collapsed.
Such is the erratic nature of the weather, even for arrays in close proximity. So we must be careful when making assumptions.
