The guns would have wowed the Japanese as they did OTL, the real question is what would occur if they simply told Perry to toss off. Perry was at a disadvantage in terms of numbers, and supplies, yet his firepower was awe inspiring at the time. USS Mississippi for example had two ten-inch, and eight eight-inch Paixhan guns. They had a range of almost two miles, and that made the Shogun with in shooting distance of the Americans in Edo.
Of course after the intial attack the Americans have to run on home. The options then are going back with more force, or doing what most other european nations at the time did, and just comply with the Bakufu's regulations on foriegn contact.
I'd see Britain joining in if America really tried to force the hand of the Japanese. I'd expect the Shogun to agree at that point, just cause much of his power base is held with in containing the ruling clans. Heck he'd have a big problem if his palace is destroyed, as by law the various clans are not required to send their familes to live in a "well defended, and stocked" section of the Shoguns palace if it is damaged.