PGR2 had a dry, wet (with decent rain effects), overcast, and nighttime version of each track (the night time ones looked really nice but the cars looked rubbish due to wierd lighting). Lots of tracks in that game, if I recall. Skyboxes take an insignificant amount of disk space, ditto to tweaks to the physics engine. Now, the wet textures are another thing. PGR2's solution to that was a general reflection mapped texture covering the whole road for the slick look. Of course, it isn't as eye popping as the puddle-strewn tracks in GT4, but at least it's an option. For example, an overcast track can seem like a completely different location compared to it's fair-weather sibling. Changes the mood and adds replay value.