Very subjective, but my MBP is a Corei7, 8GB RAM, NVIDIA GeForce 330M. With 4K footage, I need to drop resolution to 1/8 for zero dropped frames on playback and for editing.
At 1/4, I drop frames too much, so that it wouldn't be suitable for editing. YMMV with your system and the AMD card. I suggest giving the trial a spin with your footage and your computer.
Kevin Monahan
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