I’ve traveled all over with my old huge A5 haulbag (and huge Metolius and BD haulbag) and never had a problem, as it seems to just be seen as a very large holdall, and isn’t seen as ‘sports equipment’. Usually when you fly to the US you get two pieces and I tend to travel with the huge bag (put ledge in there) and a huge rucksack. The restrictions seem to be 62in/158 cm (longest side) and max weight of 50lbs/23kg (HE regs), and you either get two pieces at that (in a team of two that’s more than enough… unless you take all your water as well!). More airlines are moving to single check in bags, but even then the price for a second (or third - most allow 10!) is pretty low, and cheaper than being over the limit.
Having a very large bag can be a bonus, such as the time I fly by via Ryan air from Norway with a haulbag that must have easily weighed 50kg (meaning several hundred pounds excess charge). When I put it on the scales the bag was so big it just wedged itself between the sides of the check in desk and only the half the weight landed on the scales. The check in lady didn’t seem to notice, so I got away with it!
Big bags will always go through oversized baggage, so make sure all straps (hip belts especially) are removed or tucked away, as if it was being hauled, and it’s worth putting your name and address on the bag as well.
Way’s to keep down the weight are to avoid taking tons of camping stuff (many times I haven’t taken a stove, and just gone to the cafe or eaten cold food), and I’ve even not bothered with a tent, although when the mosquitos are out that’s not a great idea (sleeping in the boulders can also be spooky when a bear is snuffly around your feet).
Expect you’ve bought your flight, but would always recommend flying direct to San Fran from Heathrow, as going via the East coast is a real nightmare, and generally leave you knackered. Going direct you can usually leave the UK in the afternoon, and be in the valley midnight the following day - although arriving in the daytime is always best if it’s your first trip - as seeing El Cap for the first time has to be savored!