I've been, quite a few times
Its probably not what you are expecting at all, but these (what you assume to be huge) Japanese tuning companies are just tiny industrial units with a few guys working there and some nice demo cars
What kind of places were you looking to go to? Obviously you can't just walk into big companies but the Japanese are very hospitable so will normally entertain you in some form.
As Dr Doom says that Super Autobacs is definitely worth the trip as its bloody massive and you will find almost anything there. They have a specialist bookshop, cafe, and any amounts of lovely parts
Best Regards,
Gaz.