Having a lot more time than necessary for the JAA CPL flight course isn't a bad thing in my opinion...
As I started off flying privately I had four hundred and something hours before deciding to do my FAA CPL/IR... I now have five hundred and thirty odd hours and am looking at around 600 hours total time before starting the JAA CPL course (got to pass the ATPL exams first).
During those hours I have enjoyed flying, have had three great flying vacations touring the USA and a few hundred hours flying around the UK and the near continent *without* the need to hour build for certain ratings etc.
This has, I believe, made me a little more comfortable in flying as opposed to when I had only had the 150 hours required... but may I add this hasn't made me complacent.
Once I have my JAA CPL/IR I should be very near to the 700 hours total time required to apply to Air Taxi outfits... also the added experience as a "PPL" will probably make me teach people in a different way (just different, not better) as I've made quite a few of those classic mistakes people do after obtaining their licences...
The FAA IR:
On G registered aeroplanes the FAA IR will give you the ability to fly in IMC outside of controlled airspace. It will not however give you the full IMC privilages to fly IFR / IMC in Class D.
In order to obtain the CAA IMC privilages off the back of your FAA IR you will need a JAA PPL and to formerly apply to the CAA to add the IMC to the JAA PPL. If you have obtained your FAA IR within the previous 2 years you'll be excempt from the IMC Written Test and Flight Test. You will however be required to renew your IMC rating at the normal two year period just like every other IMC rated pilots.
You'll probably be better off doing your FAA IR as a Part 141 straight after your FAA PPL. (Or soon afterwards). If you did it via Part 61 then you'd need 50 hours of P1 cross country time. X-C in the FAA terms is a flight with a landing at least 50nm from the point of departure.
Hope this helps.
Best wishes,
Charlie Zulu.