I believe the Spitfire XIVe to be superior to the P-38L as an interceptor; it was just as quick to 20,000 feet (7 minutes for both, if I remember correctly) as the Lightning and would be more comfortable in and amongst the enemy escorts if they were encountered. The Spitfire had good firepower with 2x20mm Hispano Mk.II and two Browning M2 .50 cal. The only advantage of the P-38 would be the extended time in the battle, but this would not be as important in an intercept.
Plan_D - I would offer one reason why I would want the P-38L as an interceptor. Huge differences in range.
I feel the Luftwaffe failed to press US Daylight raids at earliest opportunity, even with nuisance attacks, to attempt to force long range escorts to drop external fuel prematurely. If the LW had the Ta 152 much earlier or even a P-38L with very long loiter time, they could have put them over the Channel on the Border as the bombers were forming up.. possibly even attacked from high altitude during the formation assembly process and put enormous strain on planning for 8th and 9th AF Fighter Command and RAF Fighter Command?
To me the climb rate to 20,000 feet is interesting but less important with good radar coverage giving more reaction time. In the case of US daylight attacks a simple brief burst on a radio transmitter near American airfields during engine start process would have been better than radar.. could time the Formation Assembly almost by the clock from Engine Start Time on the mission plan.
Secondly, I can't think of any bombers the luftwaffe had that didn't fall very nicely to a simple ol' battery of 4x .50s on the P-51B when they caught Ju 88, He 111, He 177, Do 217s while roaming the east German countryside.
The Spit battery of 4x20s absolutely better in my mind but the 2x20 plus .303 would be slightly lower preference for me.
Last, I like the P-38 firepower concentration factor of all 5 weapons having essentially a 24" diameter cone of fire and complete ballistic convergence from 0 feet to 300-400 yards. I suspect the effect for the me 109G/K was better than expected with the central battery, even though slowed in rate of fire with respect to P-38, by prop synchronization requirements?
Both great ships, but for Interception I would go with the P-38L over any US conventional fighter for the reasons noted...