Check out the web for a good wheel size calculator... But here's what I found by doing just that.
Top is current set up - .25 mm for your wheel spacer is just under 1" of wheel spacer.
Bottom is proposed set up VS original stock (48mm offset). But I realize I have the incorrect tire size in the "new set up" by using stock 225 vs 235.
What this shows is that by going with that zero offset wheel - you're going to push the wheels out about another inch (.89") from current and then even further - nearly 2 inches - from stock (1.89"). That can put some stress on the suspension components and also could easily lead to fender/wheel well damage if you hit a bump hard enough.
With the 2.5" lift, you've already opened yourself up to excessive stress on the suspension and driveline - CV joints and boots, axles, and more.
I'd almost want to say "the damage is already done" but ....
Just a question - why are you using the spacers with the current '08 XT rims? They're the same offset as the 2015 stock rims (alloys, not sure of steelies) so that's just pushing the wheels out an inch for no real reason - unless you're going for that "look" of the wheels being spaced wide out.
One other note - this could seriously change the handing (on- and off-road) with the big change in width/track (going up nearly 4 inches!) and the suspension geometry and steering will be very different over and above the 2.5" lift.
Personally? I wouldn't do it. But then I'm not looking for extreme off-road look or ride and my Forester is - first and foremost - my daily driver with some decent off-road capabilities for those forays into the dirt, sand and mud.