A 15 year old Goodman system that is operating well under adverse conditions...well fancy that. There are some brand bashers on this site who will swear that your system cannot exist
I would not put a lot of money or effert into any 15 year old system. The fact that this system has been operating with a leak, and is already 15 years old, tells us that it may already be operating on borrowed time. Any significant amount of money spent trying to repair it could well be just wasted.
I would definitely not put a 2-1/2 ton system in a home designed for a 2 ton system that is doing the job as is. Oversizing HVAC equipment is one of the worse things you could ever do for comfort and efficiency.
If you are having a R410a system put in, insist on the lineset being replaced and you won't have to worry that your current leak could be in it somewhere.