Im in a 2.4i and want to move brakes up from 278mm to 300mm. Caliper brackets look the exact same for the 2.4i and t5 (which is 300 mm) so does that means its an easy upgrade that will fit or am I missing something vital?

 

Yes you need minimum 17 inch rims or the calipers will touch the wheel rim , I think the 2.4 came with 16 inch rims in most countries , so you will have to change all 4 wheels , not worth it as the stopping power will not be that much better ( unless you use it for track racing)
I think they went for 300 disk on the T5 due to its top speed of around 240 km/hr obviously its only the front but you dont want different rims all around

 

Thats good to hear I was planning on getting better wheels a bit larger anyway I appreciate the input

 

Ok but before you you decide on any changes ensure that the calipers are the same as I dont think so. disks and calipers are always a pair . the 2.4 disks the brake pad uses the arear of a 278mm disk and the T5 a 300 disk so in my opinion they cannot be the same , why would a manufacture make larger calipers (major cost) and then install smaller disks?
I think you may have to change the entire calipers as well and then its really not worth it , unless you can get some 2nd hand ones

 

its a common upgrade on the 850/S70s to replace the caliper carriers to make room for the larger disks. And as noted, you just need to make sure the wheel will fit over the new spacing.

 

Ok but then having the same caliper will not improve the stopping power as the entire disk will not be used might improve cooling and look better

 

the larger disks mean more heat dissipation which translates to be better stopping power. The major cause of brake fade is the hot gasses given off by the pads during hard braking (pads are literally boiling off ) reduces the grip. Putting in larger rotors, slotted or cross drilled rotors etc or changing pad material will all contribute to reducing the heat and in the case of slotted or cross drilled rotors, allow for venting of the gasses to maintain stopping power. To that point, if you're not interested in going to a larger disk, going to a slotted rotor is an effective upgrade.

 

I was going to upgrade the calipers hence the bigger size rotors. Suppose I should have stated that lol

 

That is a substantial cost , I had a 2.4 and from what I recall the brakes were fine I would just change to slotted disks as mentioned , will reduce the life of the pads but will certainly work better, changing the entire front disks/ pads/ calipers / carrier is nor required in my opinion , but will work and look better . either way good luck hope it all works nicely for you
By the way I was told that the original 16inch rims will clear the T5 caliper set up ,just , but I have not tried it, was told by an Volvo mechanic