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for PPSSPP is an impressive feat of community modding, but it is ultimately a compromised experience compared to the official Android/iOS releases

Generally, no. The compression mostly removes "dummy data" (padding used to fill the UMD disc) and compresses audio and video files. In a highly compressed version of San Andreas, you still get the full story, the full map, and all the radio stations, though some high-fidelity cutscene audio might be slightly lower in quality to save space.