There is a huge difference between these two lenses Surrey Sam! As Rich says a 60th is almost 99% guaranteed to bring at the least a soft image - if not a downright blurred one!!
The other thing to consider with the 100-400 pump is that it has an optimum working range - as do all lenses - and as I'm hopeless at maths I can't put it down here as a formulae - but its closer than you might think - you do still need to creep up on things.....
I have been using the pump for two years now and even since getting a 400 prime (one lens between two people) means I will be continuing to use it
at least until I can find another windfall!
By all means have it checked out as the possibility of it not being quite spot on does seem rather high - and peace of mind will increase your confidence and use and help you to push the lens to its highest capabilities
Good Luck
Pauline