In summary, if your system is efficient and the block can take it! then for each 1 bar increase in absolute pressure or each extra atmosphere you will make 100% more power.
250bhp N/A on 5.6 bar absolute boost pressure = 1400bhp
1400bhp - 250 bhp N/A = 1150bhp increase due to boost pressure
1150bhp/250bhp = 4.6 x 100 = 460% increase in power! simple.
Therefor each 1 bar over N/A = 100% power increase as there is 100% more air, assuming you keep charge temps under control the motor was not built by BDC 
As a historical context in 1986 when BMW managed regularly to get 1400bhp in qualifying from their stock block 4cyl 1.5 motors they were pumping 6 lt of water per lap onto the after cooler to keep charge temperature at the normal level of 45 deg C! so their power output followed this linear scale quite well as it did on the dyno when they tested it, you will need to compressor map width and turbine to match before you think of doing shit like this, but its been done before in the rotary world.
Typically on a 2 rotor full bridge port engine a GT45R is needed to support 36 psi gauge pressures and keep the relationship in tact, delivering around 825rwhp or so on cars I have worked with. For the higher pressure ratio's in Garret Nomenclature you are then looking at GT47 sized units or hybrids to do this.
But for most people who are normal this is interesting to see your power to boost and if your set up is efficient or if you are wondering what you can do with your bits you have. There should be lots of honest 500 to 600bhp cars around, then again lol