Any question answered...

Look at cleaning the pads and rims. I would use brake cleaner to clean the debris off. Or a non abrasive cleaner such as cif. I might even give the pads a scrub with hot soapy water and a toothbrush.

I tried that ages ago, no luck.

Only two possibilities left:

1. Shimano Brake Pads are really awful.

2. Machined Rims offer massively improved braking power.