We went through all this last year with DS - he sat for WCGS & Wilson's (can't comment on the other schools) and he got a place at WCGS. He's now more than 1/2 way through the year (can't believe how quickly it has gone) and is thoroughly enjoying it. As parents, we are very happy with the school. The new h/m has made a few changes (for the better imo) which are slowly taking place and communication is very good. The school is also pushing sports again, which , imo (again), is good.
We started DS with a tutor at the beginning of Yr 5 and she focused on his English skills (typical boy - hates writing), whilst we worked on his maths & VR at home. Tutor used Bond for Eng Comp and recommended Scholfield & Simms for maths and Bond again for VR as well as Susan Daughtry. You can get all these from Amazon, probably cheaper than Smiths.
DS sat the SGS test run by their PTA (don't worry to much about results, it's the experience that counts) and he did 3 papers a week during the summer hols before the test (one for each subject) to work on his times. You can find papers on this website and try googling Dulwich and the like as they also publish test papers you can use. We had a time table so he knew what days he would be doing what which kept him happy (he's that kind of a boy).
Tutor & I (using this website among others) came up with a list of story titles and he used to write one a week - I would time him so he didn't spend ages doing it. Started out at an hour and gradually brought it down to 20 minutes (this was the time in the real test at WCGS).
DS used to do a couple of maths papers, a couple of VR papers and an Eng Comp each week (took maybe 1 1/2 - 2 hours a week ) and a story every other week. We would have week long breaks over Xmas etc (and 3 weeks when we went on holiday!) I like to think we kept it fairly balanced and it didn't interfer too much with his out of school activities.
My advice would be:
Check the websites of any schools your DS is going to sit for to get test information as it may change from year to year
Get your DS to read (as much as possible)
Keep the extra workload realistic - you don't want your DS to burn out before he turns 11
Have a back up plan
Now, DD starts Yr 5 in September so lets see if I can follow my own advice!!!!
Hope this helps