cg10's run rich with a performance manifold and de-cat on dont they? thats why cg10 micras wont pass the mot with 1 cat in, you have to have 2 cats in a cg10 for it to pass because it runs rich with no/1 cat in. were as the cg13 will pass with 1 cat in. carnt you get a fuel safc to cure the cg10 running rich by altering the fuel ratios??? then it could pass the mot with 1 cat.
The amount of fuel won't be affected by the removal of a cat. The problem with the janspeed style manifolds seems that at idle the lambda sensor doesnt seem to get as good a reading than as with the standard down pipe. In any case the lambda sensor feedback would compensate and keep the AFR steady, and compensate for increases in engine airflow to a certain extent. your car wont suddenly run rich by the removal of a cat.