It's me "shoving my nose in" again !
I don't think there are many Lexus owners who are short of a couple of hundred pounds each year. You seem to imply that I (and other auto owners) bought an automatic because of the cheaper tax. If you look closely, you will see that I drive a 250C, a convertible. This model commands quite a substantial price premium over the saloons, hardly the purchasing behaviour of someone stuck for a couple of hundred a year.
There are a lot of motorists who are motivated by cheap ownership, but certainly not most in my opinion. I consider the majority of motorists will seek value for money balancing cost of ownership against quality, performance, fuel economy, practicality, comfort and prestige amongst other buying criteria.
£200 per annum additional tax costs has little to do with the undesirability of the IS250 manual. The automatic is better in every way. Using the steering mounted paddles, gear changes are both smoother and quicker than using a clutch and a stick. In automatic mode it is as smooth as silk. Best of both worlds !
I doubt that the number of people showing an interest in your Lexus is sufficiently large a sample to draw any meaningful conclusion from.