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Paracorr-2 is a gold standard coma correcting device. Imaging AND visual. Second to none, including Keller's and Riccardi's Wynne variants.
GPU is a good choice, so is RCC1. It really depends on how much BFL you need, and how big chip do you want to cover. I would give GPU (the tiniest) edge, because Dave Rowes' design will vignette a tad more at f/4.
Another option to consider may be Harrie Rutten designed Explore Scientific HR coma corrector. All 3 of those will be similar in performance at f/4, but not in the same class as Paracorr-2, Riccardi's or Keller's Wynne variants.
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