The Bf109 is best in alt range of 0-5000

[Viper2000]

I think that the Spitfire and the 109 are close enough in performance that with CEM on the pilot who best manages his aeroplane will probably win.

The reality is that performance doesn't matter anything like as much as SA.

The 109 has a firepower advantage whilst its cannon last, and in many ways its cockpit is superior at this stage in the war for day VFR cruising & fighting. Ammunition counters are especially valuable. OTOH, the cockpit workload for fighting is somewhat higher, especially if you get low and slow due to the need to continually adjust prop pitch & two radiators. The poor rear view is also a handicap.

Later on, given +12 boost, the Spitfire especially will give the early 109 an extremely hard time below about 3 km, and I suspect that online there will either be a limitation of access to 100 octane for the RAF or else a reliance upon the artificial addition of large numbers of early 109F0 aeroplanes to balance things up, especially given the tendency of online fights towards relatively low altitudes.

Having said that however, I suspect that there are quite a lot of tweaks to be made to all the models yet, so it's too early to reach definitive conclusions about relative performance.