Which 109?
As it is, from my little bits of testing, I'd say the Spit14 turns a bit worse than 109F-4, bit better than Yak-9U. 109F-4 out turns a Typh, but Typhs out turns the 109G-2 and G-6. (In the long, sustained turns of 3~4 circles or more) I think the Spit14 turns a bit better than Typhs.
So in comparison with 109s:
E4>Spit14
F4>Spit14
G2 G6 G10
The Spit14 turns better than all other planes that go over 400mph, too, deck or at alt.
True, it does tend to become unstable when it sustains turns under 250 mph, but all 109s are much more unstable under 250mph except the E4(even the F4 starts to wobble under 250mph).
If you are out turned(flat, sustained turns) in Spit14s by any other 109s than E4 and F4, you're doing something wrong.