For T_c=293 and T_h=303, you get efficiency = 1.4% for zT=5 and efficiency = 1.0% for zT=2.5. So about a 40% relative increase as OP calculated and negligible absolute change.
Improving from 1% to 1.4% is a huge improvement. It’s 40% more cooling for a given power input or 40% more power output for a given amount of heat consumed. Alternatively, it means you consume only 1/1.4 the resources to achieve your goal.
This does not imply that 1.4% efficiency is enough to be useful for most applications, of course.
This does not imply that 1.4% efficiency is enough to be useful for most applications, of course.