(click for full size)
I think that explains a bit of the GPU heavyness, all the small black dots back there where it hasn't loaded textures (nor buildings) yet (spinned around few times to get it load everything else but that direction, then turned there and quickly snapped the shot to get it without textures) are moving cars, their physics are being calculated, their driver ai's are being calculated - that has to be heavy indeed.
Now, wether this was in any way wise decision to draw them back in the areas where you can't possibly normally see, I won't comment, but maybe this still explains part of the heavy part.
Have you tried sniping from up there? One problem with console GTA4 is that the interesting places to snipe from were SO high that you wouldn't even see traffic or people walking by.
Statistics
Average FPS: 47.91
Duration: 37.46 sec
CPU Usage: 58%
System memory usage: 63%
Video memory usage: 77%
Graphics Settings
Video Mode: 1680 x 1050 (60 Hz)
Texture Quality: Medium
Render Quality: High
View Distance: 25
Detail Distance: 37
Hardware
Microsoft Windows XP Professional
Service Pack 3
Video Adapter: ATI Radeon HD 4800 Series
Video Driver version: 6.14.10.6891
Audio Adapter: SoundMAX HD Audio
Intel C2Q Q9300@ 3ghz
File ID: Benchmark.cli