You will notice that Mr. Bingley did not arrive at the inn at Lambton with the Darcys. Mr. Darcy could have discussed Elizabeth with Georgianna in the carriage on the way, while Bingley was on horseback, following them. (A visit not granted by Mr. Bingley's sisters, who also shared the acquaintance.) I would think that a younger sister, unused to seeing her brother praise any young lady (without caveats!), would be more moved by seeing his face light up and his nervousness increase as they approached than by any simple line or two of praise in a letter from months ago. By THAT conversation, Georgianna would have been left without the power of finding Elizabeth perfect.

Do you think it was shy Georgianna's idea that she make the visit on the morning of her arrival? Mr. Darcy suggested it in such a way that Georgianna, who would willingly have put it off until she could screw up her courage, was swept along with his enthusiasm.