The carbon build-up on the intake valves is a common problem on direct injection petrol cars.
On the US forums, the IS250 has horrendous problems with this, with many people getting engine replacements. It doesn't seem to be as big a problem in this country, possibly because the IS250 is less common, but possibly because the fuel quality in the US is terrible, whereas the minimum standard for petrol in the UK is extremely high.
It's also possible it is something to do with oil - in the UK, lexus specify 10k mile oil changes and most dealers use a high-end synthetic oil, whereas in the US lexus specify a 5k mile oil change interval, and in order to keep service costs down, lexus sell a special US-edition downgraded oil filter, and most garages end up putting the cheapest possible oil in.
Intake valve deposits are something which is now a specification for new oils - so the oil has something to do with it, and the latest specifications e.g. API SN do require the oil to be resistant to deposit formation. So, I try and use a fully synth SN grade oil where possible (I use the cheap Euro car parts Triple QX 5W30 C2).