Several US fighters imported into Europe early on showed substantial performance loss due tot eh need to fit armour, armoured glass and self sealing tanks....... the original P-51 being a case in point.........
What tends to be forgotten is that the P-51 started life as a British aircraft, albeit designed and produced in the USA. It was built to specifications laid down by the British. No modification was necessary upon arrival in the UK. The P-51 came with all the items you list
Harry. Windscreen was 1.5 inch armour glass,the pilots seat had a 5/16 inch plate from just below the seat to a point level with his shoulders and a 7/16 inch plate above this to protect the head, a 3/8 inch thick firewall, armour plate forward of the coolant tank, and self sealing fuel tanks. You can see the British bought their combat experience to the design.
Early versions of the P-51 had four 20mm cannon, a report written on 30 December 1942 recommended changing from the 20mm to .5 inch because,
The present armament is considered adequate, but is functionally unsatisfactory. It is believed that four .50 cal (high rate fire) guns would furnish ideal firepower.
It doesn't spell it out, but infers to me that stoppages were a problem.