A properly installed tub spreads its load out over a fairly large area, so if the floor is up to specs, reinforcment probably isn't needed. Homes are generally built for at least a 40#/sq ft load. Now, if when they did the plumbing, the chopped through joists against code, it could be an issue. The bigger weight is that a larger tub will hold more water.
Also, ideally, and to provide the longest, reliable life without hassles, it's good to set the tub in something like mortar, which supports it well, and prevents flex (not necessary on CI, but good to even out the load) and prevents those squeaks.
When I remodeled my MB, I used a tub from www.jasoninternational.com. This is one of the son's of Jacuzzi, that broke off and started his own company. While most of the tubs are listed as air or water jet, they can also be ordered as plain soakers.