It is both possible to refit
a boat better than new, and worse than new, and sadly the amount of money
spent is likely to be similar if the work is done well.
I kow an Irwin
54' that since Katrina has had over $750,000 spent to refit
her, and still needs a lot of interior
work. The cost to buy a good used one would be around $350,000.
I also know a few J boats that have had extensive modifications that really do make the boat safer, faster, and all round better.
On the other hand I kow a guy who added a hard enclosure to an Alberg
40ish that weighed so much the boat became unstable, and couldn't be sailed anymore.
Truth be told there are likely more remits that damage the value of the boat than ones that help, but there is no hard and fast rule
I have ever found that defines it.