We know how fast a 1000hp Bf109 would have been, because the early marks were that fast. They were too fast to plink, even before getting into the issue of the MK103 being unable to work in the Bf109 historically and the earlier and bigger MK101 being even less likely to work; the issues wasn't that there wasn't enough space for the barrel to fit between the engine so much as the gun was simply too big to fit in the fuselage of the Bf109. If the Bf109 sacrifices speed to shoehorn the huge cannon somehow in the fuselage sacrificing speed for being able to fly low and slow like the HS129 minus the armor it would be mincemeat for enemy ground fire.
I'm not sure how many times it is needed to repeat:
a) that Hispano engines were with penty of space within the Vee, the DB 601/605/Jumo 211 were not.
b) that an even faster Fw 190 was tried as a tank buster, but with a less accurate rockets and probelmatic (doe to being wing-installed) MK 103
Depends on the quality of pilot necessary. High skilled Bf110 pilots were not expendable for such a limited goal (though they were sacrificed en masse in August 1941 anyway to no gain), marginal pilots who could fly trainers and with it the easy to fly HS123 could be used for such a mission. If the plan is made early enough they could train up the barely adequate recruits on an abbreviated training program for use with ground attack aircraft like the Hs123 where they could afford to be lost in significant numbers. That is what the Luftwaffe did IOTL from 1942 on with the Nachtschlachtgruppen, they took their worst aircraft and pilots and had they fly harassment missions where both the aircraft and pilot were expendable. Payload is less important than accuracy and 450kg of payload is plenty for attacking the radar sites with ground attack dive bombers. Yes multiple hits with a 20mm with wreck even a Sturmovik, but how many of those did the Brits have around radars in 1940? They were retraining the bulk of their low level AAA to defend airfields and cities and issuing Lewis guns to make up the slack because there were not enough to go around of the heavier weapons.
http://ibiblio.org/hyperwar/UN/UK/UK-Defence-UK/UK-DefenseOfUK-IX.html
Plus they'd have dedicated aircraft to napalming/dropping incendiaries and cluster munitions on AAA to suppress/eliminate them and let other aircraft attack the target.
... but then the biplane would've been the best tool vs. the best defensive air force in 1940. That would've been using marginal pilots to knock out the crucial item of the RAF FC network. With napalm that is not invented yet.
We know how well served the Luftwaffe the neglection of training, now let's throw the pilots with small hours into the meat grinder.
The bolded part is wrong is so many ways.