I went from a 2008 15" 4gb MBP to a 2011 13" i5 4gb MBA a couple years ago. What is the spec on your old machine? Mine was core2 duo 2.4ghz. The MBA was about twice as fast for CPU bound tasks. Of course the SSD was way, way faster than the slow hard drive in the old machine, so the overall impression was that the 2011 MBA was substantially faster than the 2008 MBP.
I ran Logic Pro on the 2011 MBA and it worked great, but I only work with a small number of tracks. With an inexpensive 2 channel USB interface I was getting ~7ms roundtrip latency. I later upgraded to a 4 channel USB interface and latency increased to about 13ms.
I have now upgraded to a 2013 MBA with the i7, 8gb and 512gb and it's another step up (latency with the 4 channel interface dropped to 8ms). But the bottom line is that the MBA you're considering will be a huge upgrade from your old machine.
One thing to consider - what version of MacOSX are you running on the old machine? If the new MBA has 10.7 or newer on it, some of your old software may no longer work. Apple dropped support for "Rosetta" in 10.7 and that was needed to run old software originally written for the power pc platform.