I've tried the dongle and it doesn't work. My bike isn't capped at 16mph. My old agattu was 'supercharged' and gave noticeably more assistance but the motor started making a loud grating noise
I think part of the problem here is that nobody knows what version(s) of Kalkhoffs Agattu you're talking about. Your first post says "original Kalkhoff Endeavour Agattu 1"... but "Endeavour" is one range of Kalkhoff ebikes, and "Agattu 1" is a totally different range.
Kalkhoffs current series of Agattu 1 ebikes comprises no less than seven different models, using 4 different motor systems between them - Bosch Active Line, Bosch Active Line Plus, Shimano E6100, Shimano E5000.
Older versions of Kalkhoffs Agattu used Panasonic motors, then their own disastrous Daum/Impulse motors, before switching to Conti for all of a few months, and now Bosch and Shimano.
Without knowing what the heck you're comparing, I think it's impossible to comment.
All the motors have different characteristics/power outputs. Your original Kalkhoff may even have been a "Speed Pedelec", which generally aren't available in the UK unless specially imported.