If you meet the requirements (such as FAA cross country time, required cross country flights, night etc...) and can accuartely fly chandelles, lazy-8's, 8s on Pylons and can fly to the expected commercial standard as detailed in the practical test standard, then it should be relatively straight forward. You'd probably be looking at 10 hrs to convert the SE CPL as a rough ball park figure.
The CFI is another matter though, you'd have to do the initial CFI flight test with the FAA (NOT a DPE), which is a good 7 hr day. You'd need to pass the FAA writtens, as well as receive the required training and endorsements. Assuming you have all your lesson plans and everything, by virtue of your JAA FI then I'm sure you would find this straight forward, but the FAA would expect you to be competent and things like airspace and procedures (I assume you have not flown in the USA before?) would have to be known extremely well. Again, difficult to put a figure on it, but I reckon you could be looking at 20 hrs - some of which you might be able to do at the same time as the CPL...?