To compete, GL1800 engine doesn't need more cc's. It needs a redesign. At 72 mm × 67.5 mm bore/stroke, K16 is considered a long stroke engine, but it is still very over-squared compared to GL's 74 mm × 71 mm bore/stroke. Up to 5000rpm, the GL runs neck to neck with the K16. Above that, the BMW's better volumetric efficiency and higher compression (12.2:1 vs 9.8:1) gives it a top range that Honda doesn't have.
Honda needs to get rid of the lumbering 2-valve engine and drag its knuckles into the 21st Century. But, or course, that costs R&D yen. I suspect Honda sees the big luxury yachts as a dying breed and doesn't want to sink any more into it than necessary. They figure most GL pilots rarely rev past 5k anyway, so why make it do things most owners won't take advantage of. It's one thing to bench race with numbers and charts, but out on the open road, it would take some serious thrashing for a K16 to pull away from a GL. And that's just not how these beasts are ridden the vast majority of the time. And yes, I have ridden with Goldwings. Even at a spirited pace, there is little need to venture upstairs.