I researched the cost of third-party extended warranty coverage before buying my 330 in 2004. The deal the dealership offerred me was very close in price, and I negotiated a couple of extra years into it at no extra cost, I purchased it. I have never had a question or a problem with using it - the Lexus dealerships have all that info in their computer, and I've never been charged $1 for any of the minor repairs necessary.
Whether you would have that level of seamless service with a third-party warranty is something I don't know.
I was told by more than one person that the rear main seals are the most likely thing to need replacement under an EW, and the cost of that alone will pay for the coverage.
Where does one find Lexus warranty discount coupons? My local dealer has a take it or leave it Lexus financial set of warranty packages. Are you suggesting that the addition of a coverage can be negotiated?