if the 20ohm filter will be connected to 50ohm system (which is must probably will be), then:
1-mismatch loss will happen, and the loss of the system will appear very high, which could be unacceptable (check your specs, and overall system gain/performance requirements).
2-The filter will not behave as you see it in your simulation, becasue it is not presented with its optimum impedance (20ohms), but with a different impedance (50ohm, from the system)..so it will change its response...
In short: check what system will be used (usually, 99% is 50ohms), then: NO, you will not get what you see in simulations.
If you design the filter with 50ohms, then simulation vs real perofrmance is very comparable. You may need to tweek some stuff after measurements if the filter in real-world is off a little bit from simulation...It also depends on your frequency..
Cheers...