No, it's not true. The 20 F number is the nominal delta T or split, the difference between the temperature of the air entering the evaporator coil and the air leaving it. If your house is insulated and doesn't leak like a sieve, you should be able to do better than 20 F split between outdoor and indoor temps. It should do at least as well as the old system, and I'd be upset if it couldn't do 75 when it's 100 out.

Phew, I see I just wrote a lot of stuff. It's a little of what I've learned dealing with an underperforming system in my new construction home. Hopefully, you just have a wrong charge problem, or a dirty filter and/or coil problem, as these things are easy to correct. But maybe this will help...

How many tons is your system? Give both the evaporator coil and condenser ratings. What exactly did you replace? If it was just the condenser, does it match the evaporator coil? That can be a problem to get right especially if you're mixing manufacturers.

You need a tech who can evaluate the system as a whole.

Starting with charge, if it's fixed orifice, he must evaluate charge using superheat. If it's TXV, he must use subcooling. No matter what, he should write both numbers down on your receipt along with temperatures and pressures and preferably verify against the manufacturer's specs instead of just using rules of thumb. Beyond getting the charge right and verifying the split is what is expected given the current conditions, which depends on the indoor wet and dry bulb temps, the outdoor dry bulb, and air flow, the main thing to look at is air flow and duct work. However, if it was working fine with the old system, and the new guy didn't mess with the duct work, the problem should be limited to what he did. Still, you should verify your ductwork isn't leaking badly anywhere along its length, from the return through the blower through the coil all the way to the registers.

What is the fan speed set to? I have a 3.5 ton system in south Louisiana, and the installer left the blower on High, which gives over 500 CFM/ton and draws air from every hole in the envelope as the return is undersized for that high fan speed. Over the course of several years, including a couple of years at Medium, I found the best setting is Medium Low, which gives about 360 CFM/ton, which works much, much better in this hot, humid climate. You need a competent tech to evaluate these things, select the proper speed, and charge the system properly for it; you can't just reduce the speed without adjusting the charge, especially if you don't have a TXV, as the reduced air flow can cause the coil to freeze up. At a minimum, he should measure external static pressure and look up the nominal CFMs for the chosen fan speed in the manufacturer's literature, and he should write those numbers down for you. (Good luck with that. Out of a half dozen techs over 5 years, none of them looked at air flow, which is incredibly important, beyond sticking a hand up to a register, which is horribly inadequate.)

Here are some red flags. If the guy measures temps by pointing an IR thermometer at a register, he doesn't know what he's doing. He really doesn't know what he's doing if he stands under the register for several minutes waiting on the IR thermometer to read lower and lower. To measure temperature at a duct, you need to stick a thermometer through the grill into the duct. If your ductwork goes through unconditioned space, i.e. a stinking hot attic, and it isn't insulated extremely well, at least R8, it is likely picking up considerable heat, which makes measuring the coil split inside the house problematic; you could observe (say) 12 F or 15 F during the heat of the day when it's really 20 F. The best place to measure it is through holes in the plenum or ductwork on either side of the evaporator coil, which will give you a true reading. If it's 20 there but much less as measured in your house, you have a duct insulation problem and/or airflow problem; the lower the airflow, the more heat you pick up.

Good post Tima9.., summarized everything about AC for a home owner, may be a little too technical, but it will be nice to know and understand all these details.