Yeah 1280x1080 is perfectly sufficient for this game. Some pixel count comparisons:
SD or better:
720 x 576 (PAL DVD) = 414,720 pixels
1024 x 600 (COD4, COD:WAW, PGR 3) = 614,400px
1138 x 640 (Halo 3) = 728,320 px
1024 x 768 (MGS4) = 786,432 px
Although MGS4 exceeds 720 lines, it is does not qualify as HD due to a lack of horizontal resolution.
HD:
1280 x 720 (720p) = 921,600 px
1280 x 1080 (GT5) = 1,382,400 px
1920 x 1080 (1080p) = 2,073,600 px
Very few games actually run at 1080p it should be noted. Virtua Tennis 3, Fifa Street 3 are examples, but they lack any AA and graphically aren't great anyway.
So really, GT5 is going to be exceptional in terms of resolution and overall visual quality. Few console games will ever compare until the next console generation I reckon.