I’ve owned four 3rd generation IS. Two 300h (2014 & 2019), a 2015 200t and a 2014 250.
I never got great economy from the 200t. From memory this ranged, over a full tank and measured from the trip computer, from 30 to 34. Occasionally I saw high 30’s but never anything starting with a 4.
The 2014 IS300H usually gave between 40 and 44 over a tankful again measured using the trip computer. Occasionally I saw 50+ on journeys where speeds were 30-50. I remember one 25 mile journey where I got 70+. Always ended up in the range 40 to 44 over a full tank though.
The 2019 300h was about 2 mpg better per tankful than the 2014 version I had. Some of this may be my driving but there is no doubt that the facelift IS300H cars have better power delivery with more assistance from the battery/motor. I’m sure outright power and torque is no different but the delivery is better which in my experience led to better economy. Improvements made to the steering and suspension are also worthwhile with a bit more comfort but no loss of handling.
The IS250 economy was pants and almost always averaged no better than 30.
I’d reckon on you getting a good 10mpg better with an IS300H in like for like driving compared to your 200t. Possibly a bit more with a facelift car.
You will notice quite a big performance in performance so you’ll need to drive a few and determine whether this is a price worth paying for better economy.
Is better economy your #1 priority or does performance rank above this? I suspect from what you say that performance ranks high in what you want from a car.
They are great cars though. To be honest, I only swapped my 2019 IS because I’d had two bird strikes which cost a bit to replace various front grilles (front is very low), and a buckled 18” alloy wheel (many water filled potholes where I live) and catching the underside of the car on some raised ironworks on a new housing development. I came to the conclusion it was the wrong type of car for what I need given where I live and what I use the car for. Otherwise I’d have kept it.
I’ve gone on a bit, sorry, I reckon you’ll see a good 10mpg improvement and likely a bit more from a facelift car. You’ll lose some performance though. That said, there isn’t much wrong with an IS300H in Sport, especially a facelift version. Best way to tell a facelift version is they have no front fog lights. I think 😉