Isn't the SRAM Red cassette machined from a solid block? Can't change the sprockets on that.
I used a 42/52 12-23 8 speed on my first alloy road bike, found it hard going and moved to 36/50 12-25 10 speed on my current bike.
my derailleur will take an 11-27t but £40 for the cassette to get a couple of gears is not cost effective.