LGX versus LGZ engines

The LGX develops maximum torque at 5300 RPM. That seems quite high (well compared to the old pushrod GM3800 which peaked at 3600 RPM). There is a sister engine the LGZ, for heavier vehicles with maximum torque at 4000 RPM. So what is the downside of tuning the engine for torque at low revs? Does fuel efficiency (MPG) suffer?
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