Just using qualitative arguments, the efficiency should go down.

If the return line is not insulated, the refrigerant returning from evaporator to the compressor will absorb some heat, more heat than it had at the outlet of the evaporator. This extra heat will also be present in the compressed refrigerant going to the condenser, downstream of the compressor. Therefore, the condenser would have a more difficult time cooling the refrigerant before, the expansion valve.

So, that means you need a bigger condenser, or you end up with somewhat warmer refrigerant downstream of the expansion valve, which might mean you need a somewhat larger evaporator.

Intuitively, I think you can expect the efficiency of the system to go down. 

Hi  Sosimo, 

additional to the answer of Mr. Manfredi, if the pipe from the evaporator to the compressor is not insulated, the refrigerant will absorb addition amount of heat. Accordingly, the volume of refrigerant entering the compressor will increase. Therefore, to have the same pressure ratio across the compressor you need to apply more power in the compressor which will affect the cycle efficiency badly.     

Hi Sosimo,

Just to correct the way of presenting the question. Any heat pump system, such as the air conditioning system, is evaluated using the COP (Coefficient of performance) which normally higher than unity (or 100%) instead of the term 'efficiency' which used as an indicator for heat engines evaluation.

What has been explained above is right. In general, reducing the heat losses (and of course the mechanical ones) improves the system performance. The insulation plays a vital role in such regard. 

kind regards

work of compression will increase. Also condenser work will go up.

Efficiency decreases due to heat gain in the return lines.