The most famous example would probably be the Tizard Mission whose short desc on what was transferred:
The shared technology included radar (in particular the greatly improved cavity magnetron which the American historian James Phinney Baxter III later called "the most valuable cargo ever brought to our shores"), the design for the proximity VT fuse, details of Frank Whittle's jet engine and the Frisch–Peierls memorandum describing the feasibility of an atomic bomb. Though these may be considered the most significant, many other items were also transported, including designs for rockets, superchargers, gyroscopic gunsights, submarine detection devices, self-sealing fuel tanks and plastic explosives.
There was also significant contribution from Britain on the Manhattan project as they fused their own atomic project with American efforts. I also seem to remember that UK had many of their warships like HMS Queen Elizabeth in Norfolk, Virginia and that US got quite a lot of knowledge from inspecting those ships.
These things take years and often decades to properly research and develop, so this must have accelerated US advancement quite a bit. How much was it was the question I was pondering.
I know next to nothing about the transfer of military technology from the UK to the US. But I can correct one of the mistaken assumptions here that might undermine the question: the UK was not the "leading economic power" prior to WWII - that wasn't even true before WWI. The United States overtakes the UK in per person income sometime around 1880, and maintains a lead of 10-20% in the early 20th century, though varying from decade to decade. The 1920s favour the US by around 40%, but the great depression brings the two countries back to parity, at least for a few years. But the US recovers, and a 10% gap reappeared by 1939. During the war, the US pulls far ahead, leaving a gap that has never entirely closed since.
So far, so close, though the US is the richer of the two countries. However, the capacity for production is reflected in absolute size as well as per person income, at least if what we care about is relative power. The US' population catches up to the UK's in the 1850s and expands rapidly from then onwards. This, combined with income catch-up and overtaking, means that the overall GDP of the United States was larger than that of the UK from around the end of the civil war. By the eve of WWII, the United States was about 2.5 times the size of the UK, 131 million to 48. Multiply that by the per person income gap, and the US economy is nearly three times the size of the UK economy in 1939.
The United States is the largest, most productive economy in the world going into WWII, and the absolute gap with their nearest rivals, Britain and Germany, is such that the US could outproduce both of them put together. This is not to say that technology transfer between Britain and the US was not a relevant factor in the US' postwar economic dominance. But it cannot explain the overall shift in the balance of economic power from Britain to the US, as that had already happened sixty years prior.