I would expect to find that with the two rear cats removed that you are probably going to see a drop in mpg and wouldn't be suprised if she starts to throw errors up and run like a bit of a dog.
if you remove the front two cats as well your engine management is going to go really bonkers on you and it would harm performance as a result one would suspect. should also add that with the front pre-cats still in place your car will pass an MOT on emissions, remove these and its highly unlikely it will pass.
you maybe able to get a remap to sort this out but not sure if its possible or not, post in the tuning section for better advice.
you can get equal length decat front manifolds from somewhere am sure of it but expect a 4 figure price tag.