A pedant writes: you're possibly thinking of the C9 class, a pair of ex-NER C7 Atlantics rebuilt with an articulated booster bogie (and arguably a 4-4-4-4T!). They were converted in 1931, had the booster engines removed in 1936/7, and were scrapped in 1942/3. Gresley rather liked booster experiments: earlier, he had tried fitting a booster to the rear carrying axle of an ex-GN C1 Atlantic - indeed, the project started before Grouping, with the modified engine going into service in summer 1923. That experiment didn't alter the wheel arrangement, though, but the booster wasn't removed until 1935.
As for streamlining, Locomotives Of The LNER has a general arrangement of a proposed streamlined C1, dated Feb. 1936.
All the people who said above that Raven as CME would have meant mainline electrification sooner: remember, Gresley started electrification as soon as he could get the money for it. Which meant: when the government was willing to fund it, because the board certainly wouldn't. All this speculation about different possible CMEs tends to gloss over the company directors' role in company leadership. The LNER was lead by Whitelaw, who had already shown his parsimonious nature before Grouping (hence why Gresley ordered an extra batch of D11s, because he needed something quick to sort out the state of the ex-NB area's traction). If someone else had been elected Chairman - for example, if Sir Frederick Banbury hadn't been so anti-Grouping that he refused to become an LNER director - then things could have been very different again.