Then surely it is better to under perform in the sats tests as you'll get more help/interventions whatever your ability?
That isn't how it works. If you under-perform in SATS, your target grades will be lower. Consequently, if you continue to under-perform at secondary, you will be deemed to be fulfilling expectations, and no intervention will be offered.
In contrast, if you do well in SATS, your targets will be high and expectations will also be high. If you don't seem to be meeting those expectations, additional support may be offered.
Take my dd, for example.She did exceptionally well in KS2 SATS, and is therefore targeted to get a 9 in every subject at GCSE - including non-academic subjects like PE and art. She continues to perform very well in the academic subjects, so there are no issues there, but she is currently deemed to be underperforming in art and PE, because she is distinctly average in these areas. The point is, she is offered intervention and support in these areas because she is expected to do better, whereas other "average" kids are not given support because they are performing as expected.