As you look at the graph notice the percentages of the 'islands'? The smallest one in the middle shows a efficiency rating of 75%. That is the most efficient this turbo is.
So if you boost it to 2.5 bar (22lbs) then the EF rating drops to 72% but the turbo, which normally spins at 150-170000rpm, is now spinning at 210000rpm. That's gonna shorten its life. Plus at that speed more heat will be created as it compresses the air (hence less efficient) which will affect other things like what timing you can run.
There is a lot more to it than just that though.