Putting SSIS on it's own box has many benefits, mostly by distributing load, in fact it was designed to be used this way. Hopefully though you use dataflows and not Stored procs on the server to move data, which I have encountered before!
That's what I thought but one of the dba's here was worried that it would double the network load in that it would first have to be copied to the SSIS server before it was then placed on the target server. That's why I posted the question in the first place.
I have subsequently tested this informally, i.e. executed the packages on a 2012 server to move data from a 2005 server to a 2008 server, and did actually see a slight performance improvement in terms of execution time. I'm not going to say this is why performance improved but I am satisfied that performance did not degrade dramatically which was my concern. I'm going to, when I get a chance, create a cleaner test scenario in a dedicated environment.
"Beliefs" get in the way of learning.