Spa bath pump sizing
Are you planning to build an entirely custom bath unit? Or modify an existing product?

Most jetted tub manufacturers I've come across explicitly require certain pumps and void warranties if third-party pumps are used.  For example, American Standard brand has the following in their installation doc: 


  Do not make modifications to the
  whirlpool system or remove pump from
  factory mounting. This could adversely
  affect the safety and performance of
  the whirlpool and void the warranty


I'd check with the manufacturer first before going down this route.  If they do support it, they could probably make appropriate recommendations.