If the engine runs full throttle in a standard attitude and then leans when held nose up I still suspect you have a feed issue, one that hints at inadequate muffler pressure or a deficiency in the carb to draw fuel under the higher demands created when the tank is dropped a considerable distance below the carb inlet.
Not sure about the plastic tubing being at fault as any flow restrictions would be present regardless of attitude, (meaning a condition where the lines cannot flow enough to meet demands would be evident at all times)
I've been running plastic tubing for nearly 2 decades now, all the way up to a Saito 150 with no problems. If checked against the aluminum tubes supplied with some ARF setups the ID is pretty close to identical.
Also would be leery of warming up the rear cover enough to flow out the lead as that temp may be close to the melting point of the alloy, as well. May require a bit of drilling around the periphery close to the cover and then trying to pry out the inner chunk. Any remaining lead may then come away with a bit more work.
Sorry, very tired and may not be explaining things clearly.