RTWFF, as you said "I'd like to renew Silver ", I have presumed you are Silver now. If not, then simply deduct the Silver bonus mileage...
SYD/BKK - 4,684 base miles plus 1,171 Silver bonus = 5,855 miles & 30 SC's
BKK/LHR - 5,928 base miles plus 1,492 Silver bonus = 7,410 miles & 40 SC's
LHR/SFO - 5,350 base miles plus 1,340 Silver bonus = 6,690 miles & 35 SC's
SFO/SYD - 7,420 base miles plus 1,860 Silver bonus = 9,280 miles & 45 SC's
So all up - 29,235 miles and 150 SC's on that itinerary if already Silver, or 23,382 miles and 150 SC's if Bronze.
(Assumed that the LHR/SFO/SYD are on the QF codeshare flight numbers, or the QF
SFO/SYD that only starts March 06)
Personally, I'd look at an
AA Platinum Challenge for that Itinerary (I'd assume it's an LONE4 fare) - in that situation, you qualify for Plat status by the time you've reached LHR (4,684 EQM's SYD/BKK + 5,928 BKK/LHR =10,612 EQM's, which is greater than 10K EQM's required for Plat). That BKK/LHR flight will give you 100% bonus mileage and every flight thereafter will be earning at Plat rates (albeit L class mileage is only 50%, but 100% bonus = actual mileage flown is points earned).
Would also depend on what "soon" meant - do it before the end of the year and get Plat status until 28 February 2007; do it in the first half of 2006 and you'll only get it to the same date...
Lounge access, better redemption mileage, - does that interest you...?