speeding up gear change

[Wolfracer]

Pulled the fuse, and first to second to third are all great... however at WOT, third to fourth is shockingly slow. Its almost bouncing off the limiter before it changes.
I was at Shakey's at the weekend and it was great apart from that final change which either manually or letting the autobox do it was labouring something chronic.
Any tips to speed it up, or is it a box tune?