You'll have to tell dvla about changing to a 2.0 litre and then tax increase if you go k20. I think it's personal opinion obviously. If you want a lot more power and torque a turbo can be your friend. I'm not sure what a tuned k20 can get mpg wise but mine manages 30 mpg if I drive carefully.
Si that rule only applies for cars registered after March 2001 and then its done on CO2 emissions rather than CC. A K20 swap will not cause a road tax rise.