I was also going to comment that you can get the stopover in New York for the same 20,000 AA miles. You will only pay an extra $2.50 fee.

I am also planning a trip to Denver - to buy outright a WT+ worked out at £895, taxes included.

To do Cash Outbound WT+ and Return Upgrade to Clubworld would be £945 + 12,500 Avios.

To do Economy Cash + Upgrade to WT+ with Avios would be £950 + 25,000 Avios. .

Here is something to consider - If you transfer 7,000 miles from the AA account with 28,000 (i think this would cost $100) to your 43,000 mile account you then have the following:

50,000 - good for One Way Business Class (which if your wife uses to go LHR-JFK, she will get the New York-Denver trip at no added cost in 1st class domestic).

21,800 - good for One Way Economy AA

Then perhaps use the Avios for the return - again with the sale fares and the high taxes/fuel surcharges you need to do this carefully, as otherwise you could waste your Avios.

Maybe consider buying the airfares and using avios for hotels in Denver - not the best way to use Avios, but a thought.

Good Luck