I don't see how the capacity to earn another 125K bonus MR points over Plat makes the extra cost of Centurion worthwhile, or is even of much value in itself.
Unless you are absolutely determined on MR points you can get the FF point equivalent at the same MR points for nothing or a small fraction of the price by using a bank issued Amex card as a backup to your Plat Amex.
That's what I do; between my Amex Plat charge and Amex Plat CC I have $300,000 worth of spend at the full rate and then my CBA Diamond Amex gives me another $333,000 spend for nothing (free card) at $1 = 1.5QFF. I use the Plat charge for expenses where I specifically want it to be on the Amex because of insurance, extended warranty or special Amex offers and the CBA card for the rest.
Qantas points have considerably less value to me than some others. The value of Membership Rewards is increased by being able to move points where I want.
Qantas is culling first class on all but the 380, which means LON/SIN/HKG/LAX. It's good for those destinations, but for others, another carrier is needed. Qantas route/aircraft changes over the next few months will retire 6 747s, replacing the mid-hauls with 333s, with no first and coughpy business. In addition, Qantas first SYDBKK requires 180000 points. The same on TG using SQ points costs 140k - a 40k saving. QF first SYDLAX costs 288k points and about $800 in taxes. UA first via SQ points costs 200k plus $200. A saving of 88000points and $600.
Our last trip SYDROM in TG first cost 460k points through SQ. The same on Oneworld would be, at least 768000 points (enough to go SYDTYO twice in J) plus the $1000 tax savings.
These examples are not to show that once of us is more correct, but currently, the choice Amex provides makes it considerably more valuable, and better value, than any bank issued cards.
Being able to choose which carrier to move points to, and which to redeem on saved me around 300k points and $2000 in taxes for the 7 first class trips redeemed, last year. I can, through my business, convert the 300k points to $3000 (or use them to fly or stay), but the saving over QF points in 2011 paid for Cent card before any other benefits are included.
I guess we all calculate based on our needs, but you say you get 'full points' on 633000 of spend. Using our Amex card suite, not including bank issued cards, we receive bonus points on 1,200,00. Obviously, we spend more on the cards than you would, but we also earn 283500 more points for the same spend. Those additional points will get me to BKK twice in first class. That alone is worth the price of Cent.
It all depends on 1/ your annual spend, 2/ how much you travel. I doubt there is a single, best answer. I accept that Plat is best for most people, but Cent works for us, in our unusual business card spend.