No. The userbase is smaller, Others would be GT5s best market and it is not coming out of the recession as quickly. Racing sims aren't as big as they were back in the GT3 days if other franchises are an indicator. The dropoff in sales for GT4 may be related to the PS3 launch somehow or it might have been the start of a trend.
I think my 10.5m guess for GT5 LT will prove optimistic while 12.5 for Reach is conservative. Why would it sell less than H3 again? I don't follow the logic. There will be roughly 17 million more potential buyers for what could be Bungie's last hurrah.
PS3 fans are quick to quote previous sales to back up their guesswork but for a Halo title sales mean just the opposite. If you honestly think ODST or HWars have burned out the franchise or it's fans you are in for a surprise.