Maybe because it's a different package, but you speak of fuel economy, just do some simple maths to see how long it'd take you to recover all that extra money you'd spend. A Mark II is a good car to own, and maybe yours is detuned to do so much on fuel, but in any case, you're losing a good car. If you were moving to a Mark X now, that's more worth it. And yes the engines were designed for 100 Oct so may not perform too well here.
No I compared a 2003 GX110, that's what you said you have. To lose an anniversary package Mark II and go for a common Allion would be a real blunder, but your car, your choice anyway