You seem to have everything pretty much understood -- and, indeed, Golds are pretty much top of the chain for upgrades W->J. Just a few comments/corrections...

- A longhaul return in W will earn you 150 tier points, while 3 or 4 will net 450/600 respectively. Flying to the US and taking advantage of ex-LIS fares (hardly more than W), and it's 280 in J (LIS-LHR-SFO-LHR). Add in some work flying (full fare Y only though) and silver should be easily attainable.

- Add a DONE4 and a European address- BA Gold is well worth having.

- If you're flying AA internally you obviously won't earn tier points on many fares, but BA silver/gold will allow you AC/FL access, unlike AAdvantage equivalents. Definite benefit.

- It's actually 25,000 miles to upgrade a return ticket, not just one way, so it's W fare + 25000 miles = J return. The news gets better