I think the TD04 is 'in its prime' around 1 bar (which suits a standard engine well) and kinda hitting its limits at 1.4 bar. TD05 shifts it all up, so it'll run best at over 1 bar (?) and go up to 1.6-1.7. Obviously you're pushing it by running it at over 1 bar on a standard engine, but under 1 bar, the TD05 likely won't really have woken up so it's going to be alot less driveable than the TD04 i.e a lot more laggy for no real gain. For bigger power on a forged engine then you'd be better with the TD05, but the TD04 would work well up to around 1.4 bar. That's the mega simple explanation anyway, I'm sure someone more technically proficient can give you extra detail if you want it!