Good on you, but prepare to step into a world of disappointment and frustration 
It's not so much a matter of hardware limitation but game engine & coding limitation.
Sticking in extra RAM or overclocking your hardware is not a solution in this equation.
V, as did IV (taxi bug anyone?) just can't handle too many replacements...
atm I've only 'replaced' the police with HQ models and that worked out suprisingly well tho.
It's not a real replacement as I'm using a dlcpack addon and let those models spawn via popgroups and dispatch.meta...
But I figure once you start replacing 1MB models with 3 ~ 6 MB ones real soon you'll get texture glitches, game running out of memory, etc, etc.
Eg when I spawn HQ carmodels (10 ~ 20 MB) I get random green flashes on the screen when driving.
When spawning different models while playing will get me a feature where whole map objects disappear and textures only render lo-res.
Usually time for a quick restart of the game 
What you describe here, swapping the tailgater for RS6 and then seeying it all around you could very well be a good example.
In my book that means the game engine decides to spawn that one as it's already in memory instead of spawning many other models.
So, already a way for the game engine to limit itself in memory usage?
