As a £7.50 Goodybag buyer this has made my day! since I tend to use most of the 250 mb (just when out and about) since I do most things over wifi, and I guess I'm not the only person in this position, I bet most of the £7.50 users won't go above 300 mb anyway!
Will I still get the new goody bag data upgrade from the 6th May 2015 on the £7.50 goody bag.
You will indeed.
The new allowances will be applied to your goodybag next Wednesday (May 6th), minus what you've already used.
So you'll have started off with 200 minutes and 250MB as normal. Then, say you've used 20 minutes and 50MB in this first week of your goodybag; well when the extra minutes and data are added next Wednesday, your remaining allowance would go from 180 minutes and 200MB, up to 230 minutes and 450MB on that day - i.e. the extra minutes and data in the new allowances, minus what you've already used.
Hope that makes sense!