If you use a 460cm IMCS 24-26 mast in a sail that was designed to work on a 490 cm IMCS 28-30 mast, you will get the following issues and you will need to figure out how to "tune around" these issues to get anywhere near the design performance of the sail:
The top of the sail will fall away/twist off far too early in the downhauling process as the mast is not stiff enough up in the top to support the upper part of the sail as the tension begins to develop lower down in the sail.
You won't get the full draft in the lower parts of the sail because you cannot develop enough "perimeter tension" with the softer mast.
On an RAF sail like the T-bird T-3 this will result in alot of folded/bunched luff sleeve fabric in the middle 1/3 of the rig. If you get enough wind to fully open this up (the way it's supposed to work) you will have too much shape in the top of the sail due to the mast being less stiff up there and using significantly reduced downhaul.
You can sail the 8.0 with a long extension on a 460 cm IMCS 24-26 mast but you'll never get the full potential from the sail rigged that way.
As far as how much extension you need up inside the mast, that's pretty much established for you.
All of the longer extensions have about 45-48 cm of extension adjustment and then
another 13-14 cm (HPL=14 cm; Chinook= 13 cm; Streamlined= 13 cm) of full diameter (with no adjustment holes) above the top adjustment hole.
I've used extensions fully extended on occasion, but if you need to go more than 40 cm, a mast extender is usually the stronger way to go.
A mast extender is a fiberglass/carbon section of mast about 40 cm long that you can use with a short extension.
The top of the "extender" is machined to a ferrule configuration that fits snugly into the bottom of your mast.
Here's the specs. on the '08 Tush T-Bird T-3:
Size: 8.00 M2
Max. Luff length: 4.95m (495 cm)
Ideal mast IMCS rating: 29
Mast IMCS Range: 25-29
Boom length: 2.23
As you can see from the specs, you will need a 35 cm (mnimum as it may take a bit more or less to really get your sail tuned right) extension for your 460 cm mast.
If you are a light weight sailor, you may actually like the feel of the T-Bird T-3 with the softer mast and more twist at the top.
On a similar sail that I use, the 7.5 m2 Sailworks Retro, the recommended (and sail designed on) mast is a 460 cm IMCS 24-26m, but for heavier sailors, a 490 cm IMCS 28-30 is the recommended alternate mast.
So at only 35 cm of extension, I'm pretty sure you will be safe with a 45-48 cm (Tall) extension adjusted in the 32-38 cm range.
I'd suggest getting the 8.0 T-Bird and rigging it on your 460 cm IMCS 24-26 mast and then try to borrow a 490 cm IMCS 28-30 mast at some point to compare the differences.
Since Tushingham suggests quite a range on the IMCS rating (25-29) they may have designed in some compromise to make the T-3 to work better with both masts.
Hope this helps,