GTA 5 update 1.28 was a bit of a rollercoaster for PC players. On the one hand, it added more top quality tat into GTA Online. On the other, it potentially added performance problems into everything. Rockstar has seemingly rectified the issue through a new patch, released last night. "This issue has now been resolved with a Title Update," the Rockstar support page states.
In addition, Rockstar has again addressed the use of mods in singleplayer mode:
"We also received some reports linking decreased performance when using Story Mode mods to TU 1.28. As mentioned previously, we are not actively targeting players using mods in Story Mode, nor are we explicitly implementing protections against them in-game. Our primary focus is on protecting GTA Online against modifications that could give players an unfair advantage, disrupt gameplay, or cause griefing. However, as a reminder, mods are still unauthorized and as such, Title Updates may cause Story Mode mods to behave in unexpected ways because they are not supported or tested, and players run them at their own risk."
In other words, sure, you can use mods in singleplayer. Just don't be surprised if things break every now and then.
