I don't think you can perk on speed alone, except in the case of jets. The speed of the 109s is nowhere near the fastest in the game. Half a dozen allied planes outrun it easily. It's overall killing capability (and sometimes over-use) that dictates what should be perked, or at least the perk points of an aircraft. 109K4 may be relatively fast (but that's compared to spitfires, which are notoriously slow anyways). However it's still not very nimble and not very well armed, compared to standard 6x50cal and 2000 rounds for allied planes, and 2x20mm 250rounds & 4x303cal 1200rounds for most spits, it's got a mere 500 50cal and 60 30mm. So it can't kill nearly as much. The speed helps in certain areas, zooms, climbs, etc, but it still can't wing over inside a p51's turn radius, nor can it hope to out turn a spit. So you can see while it has some speed over the spitfires, it's still by no means a "monster". I'd respect it enough to be wary, but I think it'd be on even footing with most other planes in the game.
All opinion, mind you, and only time and arena logs will tell otherwise, but that's my take on it.