Most likely you talk about the second batch of the 25 by Heinkel on his own account produced He-100 "D" variant, production numbers D-020 to D-025, equiped with 'normal sized permanent' cooler instead of the surface cooling with retractable auxillary cooler for take-off, the first batch, D-01 to D-019 were made alike.
WITH surface cooling german fighters would be ... clay pigeons as a vital system would constitute the largest thinkable target area. One shot and you would have to look for an emergency landing or bail-out place as in 10-20 (at best) minutes your engine would be without aany cooling at all.
But it would have to proove itself - aside the world-speed-record stunt - and has to be compared against the then german 'standard' fighter its production just having started :
the Me-109 E or famed "Emil".
Max speed : He-100 D
Range : He-100 D
Agility : more or less on par, maybe sligtly better Me-109 E (wee larger windspan and wing area)
Ceiling : more or less on par (11.000m He-100 D against 10.500m Me-109 E)
Armament : Me-109 E (in 19139 : He-100 D 1x20mm plus 2x7,92mm or 15,1mm, Me-109 E 2x20mm plus 2x 7,92mm, though on par with the following Me-109 F)
So, in 1939 the He-100 D had the performance of the not even yet thought about follow-up model Me-109 F.
Aside from Görings personel problems with Heinkel and his and therefore the RLMs (almost ?) doctrinal unwillingness to let Heinkel produce fighter-crafts ... and the "problems" of setting up mass-production lines for a new fighter craft with the war already starting/going on (
only "sensible" way would have been building completly new line while keeping on with the production of further Me-109, maybe 'forcing' Heinkel to pay for this production facility from his personal wealth ... as 'punishment') ...
Let's assume the He-100 D with the 'normal' cooler is accepted by the RLM (and Göring for whatwever almost asb-ish reasons). Consrtruction of a production facility is allowed at about September 1939.
The 5 IOTL built - lets call them "D-1s" - would undergo a thorough 'Rechlin-testing and with another maybe 25 numbers batch of preproduction "D-1s" join some field assessment-unit along the franco-german border as well as the scandinavian campaign at the change of 1939 to 1940.
Meanwhile a/the 1st production line/facility starts throwing out production models "D-2s" or perhaps even "He-100 Es" (?) in May/June 1940 (
setting up such workshop barracks for airframes on the 'green field' didn't took much longer in the midth to late thirties, all other components came from ... subcontractors).
There might be some changes already in this model, maybe to the radiator type and its fitting/lining into the airframe, maybe another weapons-scheme (4 or even 5 MD 151/15 with more ammo ?), maybe some changes to armour-scheme (?), some other, different subcomponents ? Maybe due to test-pilot hints a bit enlarge wingspan and area, increasing agility ?
As often with the 'switch' from test to production there will be some loss of power-to-mass ratio and therefore speed.
But such a He-100 E will still - with around 600 km/h - outpace a Spitfire Mk I and be at least on par with a Mk II and being at least on par with both in terms of agility. With other engines 'in-the-pipe' as with the Mk III the He-100 will at least keep up with the former.
It will have a greater range as the Me-109 and therefore be able to stay longer in combat while sporting a heavier weaponry than its opponents.
Harder times for fighter command.
German bombers better escorted deper into Britain.
But ... we all know that Britain will OFC know long before about a will be fully able to improve the Spit and all other measures and means long before the first He-100 E might show up so that it would have no effect at all.
The Fw-190 would IMO still be created, as it exploits a different engine concept and was ordered exactly for that. A He-100 in swervice would not chnage that.