No.
The 18W adapter has been discontinued, and no longer offered for sale, so Apple's packaging is going to specify the new 20W adapter to be current (there's a pun in there for the initiated), and prevent confusion.
Technically, there is little difference between 18W and 20W. In the past, Apple has vacillated between bundling
10W and 12W adapters with the iPads. They juiced it up a bit from 10W to 12W with the iPad 4 and Air 1, but actually went back to bundling the 10W with the Air 1 before it was discontinued, then going back to 12W again. For all intents and purposes, they were treated as interchangable equivalents.
It should also be noted that these are all nominal figures. None of them are going to produce 20.0W or 18.0W exactly, for various reasons, including what the power company is supplying to the house, and how it is distributed throughout it.
So "20W" really only means it will be around 20W; it could be more, it could be less, depending on the supply, and the load being drawn (particularly for multi-port adapters). So that 2W theoretical difference is going to be further diluted by variances in actual output.
As practical matter, there is unlikely to prevent the use of the 18W adapter with the MagSafe; it will still work, but perhaps just a little slower. So, unless you're the kind to sit there with a timer, any difference will be not be noticeable anyway.