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.
The new 1833cc engine does indeed have 4 valves per cylinder, but the horsepower output is still (IMHO) lacking. I sold my 2010 Goldwing and now a GA resides in it's place. Honda dropped the ball on two fronts: horsepower and luggage capacity. I think Honda could have gotten more HP out of the current engine, but chose not to for longevity. I was willing to live with "only" 100HP, but the lack of storage was a deal killer for me. Harley still has the best luggage: top opening saddlebags and a trunk you can open with a helmet on the seat.
2019 HD Road Glide
2006 TW200 (the wife's)