If GT5 increases at the current rate of ~7 points per day, it'll gain 490point between now and release, putting it at 624 points.
However the rate of the increase should actually increase quite a bit as days go by.
To throw out a theory, there are ~10 weeks (70 days) until Japan release (saying that its within a week of US/EU release). next two weeks will be 7 points/day. The following month (4 weeks), it will average 10 points per day, and the last month (final 4 weeks), it will average 15 points per day. That puts it at an additional 798 points, totalling 932 points, which is 9 points higher than GT4.
The range at which is should actually fall is between the 624 points and 932 points. 1 point is ROUGHLY equal to 1k, although thats not a good indicator of FW sales because games that have a couple of years worth of pre-orders usually have a large percentage of people that actually forget to pick up their pre-order the first week or so of release.
My guess is that it sells at least 500k in Japan first week, with sales eventually crossing the 1M border well within 1 year of release. Lifetime sales in JP should come close to 1.5M