The GX has
very little legroom for the third row seat -- the specs say almost six inches less than your IS250 ... and I've ridden in the back seat of an IS250 and it was
not fun.

Is your wife going to be using the third row seat only for child seats or for adults she doesn't like?
IMO, a better choice for a true seven passenger all-wheel drive "luxury" vehicle is a Toyota Sienna Limited AWD with navigation with backup camera and Bluetooth, leather heated memory seats, dynamic laser cruise control, DVD entertainment and wireless headphones, 2nd and 3rd row window sunshades, parking assist, etc., etc. The Sienna AWD even has HID headlights which you can't get on a GX and even a more powerful engine with better gas mileage than its Toyota Prado/GX470 cousins.
OK, I admit I've become a little "Sienna crazed" since driving eight of us - all adults - and luggage around southern Ontario for a week in a rental Sienna last summer. It was quite, calm and comfortable -- and huge inside. For a vehicle that can carry a lot of people and stuff, the Sienna is the best I've experienced.