I hope Muddy'll pipe in, cause he will def know the details, but..
Switching to bloom nutes will up P and K in the plant, which starts a bio response in the plant to go from "veg/Grow" to "Flowers/Trichs", so it'll slow/stop veg, and speed up flowering. The yield hit is on reduced size of plant, less development of pre- bud sites, and less stretch- that can cause a light penetration problem... I've not heard of a % lost in yield, but at week 4, on a plant that would yield 4 oz, it could cost 1/2 oz, I'd guess. That guess is from mistakes I made that affected one plant over an other- no science, really.. Gut feeling.
Now, let's let a guru grade my test... lolol
Staying in the UK, Dude? I sometimes wish Canada would peacefully invade my State..