What piston springs are fitted, if any?
The mixture the engine sees is partly determined by the open jet area (in turn determined by the needle profile and piston position) and partly determined by the vacuum above the jet (determined my airflow and piston position).
One of the most basic design calibrations of the CD carb design, SU or Stromberg, is to set the piston weighting (combination of actual weight and sometimes additional spring pressure) so that the piston is fully raised only at the engines maximum airflow demand. If the weight is insufficient, the main effect is that the piston will rise too much, vacuum above the jet will be low, and the mixture lean across the range. The secondary effect is that should you ever manage to find a rich enough needle that it appears to more or less work, you will then find that once you get to a certain rev/throttle combination, the carb piston is prematurely fully raised, and it looses its ability to control, the vacuum increases and the mixture goes uncontrollably rich.
150CD carbs are just big enough for a 2L @6000rpm, with not much margin for extra flow. They’ll support maybe 120bhp without resorting to “restricted inlet” type tuning tactics.
This doesn’t automatically mean they are too small for any 2.5. If you are not expecting to rev your 2.5 much beyond 4.5 - 5k rpm they’ll be fine, provided they are correctly calibrated.
Nick