The point in your post that seems to need the most attention is the issue of the fluctuation of the gauges. This is usually an indication that the refrigerant system has non-condensible gasses in it. If this were mine, I would have the refrigerant recovered, a new drier installed. Vacuumed and recharged. What you have now is probably not functioning properly. I don't think you need a larger system, although we can't see you house and make that determination. You can have the contractor preform a heat load calculation to determine the proper size. You will need to swap both units if you do make a change. Most counties in florida are not allowing systems to be pieced together, anymore.

 

Yea I get that.. But if I'm still getting good cold air coming out then it won't solve my problem.. But I figure I gotta get it flushed.. Just trying to solve the bigger problem. I just don't see that helping..

 

Did the old R22 2 ton unit cool your house ok? If so, then you probably don't need a new 2.5 ton R410A unit. Just need this one fixed. You probably don't have enough air flow for a 2.5 ton.

 

What is your attic temperature like?

 

What you're asking is a loaded question. Many things can affect the output/cooling capability of a system and just upsizing because the house isn't cool enough is necessarily the answer.

Any number of issues can affect the output of the unit. You say you've got a 2-ton system but a good tech would question whether you're actually getting 2.0-tons of performance from the system? Please know that not only are there many techs out in the field, all with differing opinions, but there are also many people representing themselves as techs but they have no idea about the real science behind the vapor compression cycle nor how to properly troubleshoot and/or charge an AC system.

Begin shopping for a quality shop. You can find one by asking how they'll size a replacement system when you call. If they say they'll do a Manual 'J' load calculation, then invite them over to determine if your current system has problems or if it needs to be replaced. They'll know by the load analysis if it needs to be increased in size. If they answer with 'experience, square footage or by looking at the old equipment" then by-pass them and continue calling others until you get the correct answer to the question you're answering.