I agree about the need to look very carefully at the entry requirements for the Catholic schools. As already said, if you don't get a GS place or a place at one of your RC choices, you will be offered the nearest school that still has places available after everyone else's preferences have been allocated. The phrase used in the secondary school transfer booklet is "places were offered to all applicants". If you aspire to a GS place for your child then it's not likely to be the kind of school that ticks many boxes for you.
In B'ham RC schools currently have 6 categories of entrant, unless your child is a baptised Catholic then he/she will be in category 5 ("non-RC child at RC feeder primary"). Our local RC school doesn't get down to category 5 most years but this year they have (for the first time since at least 2007 as far as I'm aware - they've even offered at least one place in category 6). Birth rates were low in 2000 when this year's children were born, and that continued iirc until about 2002, so your child is probably more likely to have a chance of a place than in previous years, but I strongly recommend that you ask the schools you're considering what numbers they've taken in their various entry categories in recent years (especially for entry this year) and, if they've taken non-RC children, what distance they've gone out to in making offers to them.
Also, be certain that if you are putting multiple grammars down, you place them in "last child admitted mark" rank order. For example, if you put "KE Handsworth, KECHGS, KEFW" on a girl's admission form, the second and third entries are a waste of ink: there is no imaginable scenario, absent seismic shifts in Birmingham's demographics over the space of twelve months, in which a child with that on their form is going anywhere other than Handsworth or wherever they put fourth or later on the form.
While that's broadly true I strongly disagree with the opening sentence. If KEFW is your preferred school, by all means put it first on your form, and if you also like CH and Handsworth then by all means put them on as well. But if Handsworth is your preferred school, it should be first on your form. IMO, when it comes to filling in your form you should take more account of geography and your own (or your child's) personal preferences than you should of everyone else's preferences (which is basically what the qualifying score represents).
As to what we put: for our son, 1 - KECHB, 2 - KE5W, 3 - BV, 4 - fairly decent, fairly local boys comp we had a moderate chance of getting into, 5 - local RC comp which we had no chance of getting into, 6 - Old Swinford Hospital (slightly cheaper than going straight to the independent sector if all else had failed - I bet we were the only people with OSH as last choice
). For our daughter we put 1 - KECHG, 2 - KE5W (yes, I know - wasted choice - but sometimes it's just not worth the argument
), 3 - good girls comp we were unlikely to get because of distance, 4 - local RC again (we might even have got it this year!), 5 - tolerable comp we would certainly have got. No 6th choice, I wasn't allowed to put Sexey's School in Somerset (nearest equivalent I could find to OSH) on pain of divorce.