This is odd, i have a spit book that says at low alts the SpitV actually had a high climb rate than the Spit IX. But AH plane info says it doesn't.
Which is which?
The Spit IX was introduced in July 1942. At about the same time, the Spit V engine limits were uprated, allowing it to develop more power.
With that power increase (16lbs instead of 12lbs boost) the Spit V and Spit IX had almost identical climb rates below 10,000ft.
In late 42/early 43 the Spitfire IX recieved an uprated engine, the Merlin 66, which gave it an extra 1000 ft/min climb rate. At about the same time, some Spit Vs recieved a special low altitude only engine, which gave it a similar boost in climb rate, but only up to around 5,000ft.
In short, the only Spit V to really outclimb the Spit IX would be a special low alt Spit V against an early Spit IX.
You can see the performance of various Spits, and how they increased over time, at
http://www.fourthfightergroup.com/eagles/spittest.html