The cheap black poly tubing that you referred to above is good enough for your application. It will also be good enough if you replace the compressor with one that has more cfm.
You asked - ""How much cfm will actually get out the diffuser into the pond or is it measured in psi. The compressor dosnt state at what psi it will give 1.5 cfm.""
The Easy answer first: 1.5 cfm factory rating is open unrestricted flow.
Second answer is - air released from the diffuser can be measured as psi and cfm. Air pressure (psi) at the diffuser is mostly determined by the water depth aka head pressure. Secondary pressure increases are caused by reduced size of airline and length of airline. For YOUR listed purposes and distances, the hose diameter (1/2" poly actual ID dia 5/8")and length are mostly academic and insignificant.
A diffuser that has TOO SMALL of pores or TOO FEW pores will cause the pressure and air escape at the diffuser to be greater depending on size of holes and number of pores. Essentially the back pressure or resistance to release air will continue to increase as the holes continue to get smaller and fewer until the extreme situation of no holes/pore for the air to escape and then pressure increases to very high at the pump. Thus the ideal situation, is having a proper pore size and adequate number or pores that do not cause any "measureable" restriction to air flow at the diffuser in or out of the water. Then your only major restriction to air flow will be water depth which as stated earlier is one psi increase per each 27" ( I usu use as convenience 2 ft) of water depth or 0.5psi per foot.
Depending on the capability of the compressor to create psi and amount of air flow (cfm), the general situation is for there to be less cfm air release as the diffuser goes deeper and deeper into water (more head pressure). Compressors from the manufacturer often provide a chart,figure,graph that displays this decrease of air flow as it relates to increased psi.
Provide the water depth at the diffuser and we can estimate your approx cfm based on 1.5cfm and 10psi(10psi is standard for rotary). How big and what shape is your pond? Maybe one diffuser will ultimately produce more water movement than two smaller and lower flow diffusers.
