HOUSTON (Reuters) -Top U.S. oil producer Exxon Mobil's planned acquisition of the No. 1 Permian shale producer Pioneer Natural Resources could further restrain output growth in the largest U.S. oilfield, squeezing pipeline companies and suppliers, executives and energy advisors said. Exxon, the fifth largest producer in the Permian, has set an up to 1 million-bpd target for its operations there for 2025 and more recently pushed it back to 2027. Buying Pioneer would boost its output to about 1.33 million barrels of oil and gas equivalent per day, well above its goal.