Stuart, without wanting to sound picky, neither of those graphs show anything that is unexpected. Both are laggy turbo's and both show that on the graph.

My GT28 (which is known in the Jap world as the 'Disco Potato', due to it's fast response) wasn't on boost that early.

My GT30 doesn't make full boost until after 4000rpm, even later with the bigger turbine housing.

The same turbo on a 200SX 2.0 16v engine isn't much different, so why should a turbo like that have 2-bar at 2800rpm on a tiny 1600cc engine??