Your Return on Investment is completely linked to the initial price you pay.
You may get a better return on the Sunpower but will it be £3k more over a 20 year period – I doubt it
My advice would be to stick to a good named panel and European Inverter which should be installed sub £6K and based on your SAP figures and assuming 50% electricity usage you should be returning around 15% i.e. £800 – £900 per year.
The SAP calculation is always in your start point and if you are happy with that return, then you will not go far wrong.
I would not base my investment strategy on claims of certain panels outputting 10, 20 or even 30% more electricity when MCS has already determined that the SAP Calculation is the best and most honest measure.