8000 is still in the range 0 up to 24Mbps of course.
Whether the line can run faster when going from ADSL to ADSL2+ modes depends on the line stats, and without those hard to say much, i.e. attenuation, noise margins and line speeds.
What is most likely in your case is the Virgin has moved you off of the BT hardware one C&W LLU hardware and assigned an 8Meg profile to you. Virgin use C&W where they can as its cheaper than the BT equivalient.