I am a newly registered owner of a 1993 (L) LS400 and was a little wary about the fuel consumption thing too.
I moved up in the world from a Citroen Xantia 1.9 Turbo Diesel and was expecting to empty my wallet every day.
In reality, to make any sort of progress in a lesser car, you need to drive it with foot to the floor at all times and honestly, I have noticed an increase in spend - but not hugely significant.
I have been driving carefully though - just to see what mileage I can do on a tank.
I was once the owner of a Toyota Celica GT4 import and like the Lexus, I found I drove it more sensibly than the 'ordinary' cars I have owned - I have nothing to prove; I know I CAN go quicker than the hatchback boys so I don't even bother trying.
That racks them off more than beating them off the lights I feel!
So there is not a huge difference (fuel-wise) between a Xantia driven hard and a Lexus driven gently.
When its 6:00pm, freezing outside and everyone around is battling with their neighbour on the M6 in the rush hour to get home, it is really pleasant to relax in heated leather comfort, something soothing on the autochanger, cruise set at 70 (if you're lucky) feeling smug.
Go for it - enjoy!