Hi,
Your stated objectives are unclear. In your OP it was requested to determine whether cheap bulbs perform the same task as expensive bulbs. No colour preference or other delimiter was stated. If by the expression "do the same job" you mean "do they grow plants as well as expensive bulbs" then the answer is without a doubt, yes.
If "do the same job" means do they have the same colour, then the answer is, unlikely, as each bulb will have a different colour cast.
Or, "do the same job" could refer to "do they have the same life expectancy", answer; undetermined.
In your subsequent post you appear to have a disdain for bulbs that are rated at 8000K (or at least, it reads that way) It's not really clear what colour you are used to or are looking for. If you haven't seen any of these bulbs then how will you know whether you will like them or not? And if they are only around 10 quid then it's certainly worth trying. In any case, no one can tell you what colour bulb
you will like. A 10,000K bulb theoretically should have a colour cast slightly bluer than an 8000K bulb, while a 5000K-6000K bulb theoretically should be slightly more red/yellow. Whether this actually is the case is unclear.
If you have your heart set on 6500K then check
CP Lighting GE MQI-TD 150W 6,500K Aqua at 11 quid plus VAT might be worth a shot.
Cheers,