Anyone have any experience with the ECU chips, is it worth spending £300, does it make that much difference? Was thinking of getting the cyclone chip for my 3.9efi.

I don't know about them but I was curious and found this:

Land Rover Discovery Performance Chip - Always in Stock!

If it works it's a bargain compared to the £300 Cyclone chip, in fact for those gains it's bargain full stop, but will everything cope with it?.

mark adams does ECU chips for the lucas hot wire systems probably get upto 15hp form them in a 3.9L

eBay.co.uk Shop: Search results for Partsrange.

unless you are prepared to have the ecu correctly remapped a plug in chip is not worth the cost look at more inprovments of the induction system a new cam better exhaust removal of the engine fan will give better power gains than just a chip.
have just rebuilt a 4.0 p38a engine for a customer with a piper 270 cam and a fully balanced engine gives more power than a 4.6 on a standard ecu.
the hot wire and gem ecu can run without any major mods a larger engine with improved breathing