In exceptionally overcast / rainy years, they can underperform inexpensive, good quality monocrystaline panels, due to their unimpressive performance in low light conditions
(like happened in 2012 in North West England).
In sunnier parts of the UK i.e Cambridge, Cornwall etc outperformance of 5-10% is possible annually, due to their excellent performance in bright sunlight.
In Northwest England, Sanyo 250 HITS typically outperform Suntech STP250S-Wd+ by 5-7% in bright sunny weather / clear skies, but persistently underperform in low light.
How much bright sunny weather v overcast weather dictates the level of outperformance / underperformance between a high quality, expensive panel and a far cheaper, good quality panel.