FWIW, my take on HKS bits is as follows:
They do make some good parts, which work well and do exactly what they designed them to. They also make some parts which are close to useless in the UK, for 99% of people's cars.
An example is the infamous mushroom air 'filter'. The filtration obviously played second fiddle to the amount of air they could flow, meaning people using them were forever junking turbine blades as shit got sucked through the filter.
People don't realise that the idea is that people using these filters will rebuild regularly (i.e. in the motorsport arena).
The big twin turbo kits that HKS make for the Supra are without parallel IMO (prove me wrong?) but again, you have to know exactly what you're about and how they're meant to be used before they begin to make sense.
I can't comment on their after-sales service because I'm only using one part of theirs right now and I hate it anyway (SSQV).