That's what electrically warmed suits were for.
What should have made it exceptional at altitude, turbochargers was actually a point of failure.
Pretty standard practice in that era was to get something "good enough" and then ship it to be "broken in"
The P-38s supercharger was unmatched in its ability to provide full sea level power up to the anticipated combat altitude of 25,000-28,000 feet. But it did have teething issues which in the ETO were compounded by forced use of British aviation gas. Issues with the supercharger and British aviation fuel seemed to revolve around the intercoolers and were resolved in two variants of the fighter.
Yeah those suits worked so well. P-38 was fine in MTO and PTO. There were far better options for ETO.