Save the new budget of 4k you have now for the future. The astra can do the job for the time being and you won't get anal about it like a faster special marque. I think what the question really is, "Would a Fiesta ST or GTD keep me happy?", would an ST or GTD stop the time being sunk into perfecting anything else cheaper?
Admittedly you have been looking at the Fiesta ST for a while, ever since before i got mine, maybe you should take one out to scratch that itch again and see if you do really want it. I took one out a few times and every time I knew this was the car i wanted. It's a shame you're so far away as i'd take you out and let you have a go in mine.
I should have put the deposit and money going forward into a house deposit but i didn't...i don't regret this decision though as i'll be keeping this ST for the long haul. It ticks all the boxes and is the right car for me right now and the foreseeable future. It's just the right package of speed, cost, driving dynamics. I don't mind paying for it as my car is a hobby and not just an A2B device.