In my brief survey of UK DfE-approved (and even some unapproved) baseline reception tests, I found that the majority of them operate on a mean of 100, and SD of 15, like most other age-norm-referenced achievement tests, with the exception of the Hodder test, which appears to have raw scores totaling to 150.
Which would make scores of 149 and 150 very high indeed (+3.3 SD)--but only when compared to other early-five-year-olds.
And the survey of content, even on the upper extension sections, has quite a low ceiling for any somewhat academically-advanced preschooler, with the highest items only in the range of very simple addition and single word reading.
Keep in mind that baseline reception/kindergarten readiness testing, although being used by the UK to track student/school progress longitudinally, is designed primarily to identify at-risk children in the moment, not to accurately place or track anyone performing above the 16th %ile or so.
BTW, the UPittsburgh white paper has value as a general comment on readiness testing, but is actually quite old, as the best kindergarten screening instrument on there (and the one I would still consider the premier test for these purposes), the DIAL, is now four editions removed.
Oh, and with regard to grade-skips, I notice that the reception tests have social-emotional scales as well. Perhaps the data from that portion of the assessment (usually teacher observational, obtained after a few weeks or months in class, or survey, based on parental report) may help you in your decision-making process.
