I'd argue that power level games can be pretty equal. I mean try making some 80 power lists and you'll find they all tend to be around 1500 points. (I actually did this with 75 power across five armies and they all totaled between 1325 and 1450 points.)
Keep in mind that in some cases, some of those upgrades are locked behind increasing squad sizes, which would in turn increase power level. Then theres also the steeper power cost in certain units, like a leman russ or squad of terminators.
Points are a better indication of (initial) army balance, and a PL SHOULD be about 20 points but can easily represent anything from 15 to 25 points (and something like 10-30 if you really try). Lemme demonstrate by grabbing 2000 points lists posted semi-recently and converting them to PL (the lower the more stuff you get, effectively):
- 98PL -- (with 143 points left over, so 100+)
2k Space Wolves
- 96PL (NB: Index Xenos list)
- 100 PL
So yeah. Equal in points yet we've got a fairly wide spread in terms of PL. Still better than I anticipated, I'll admit, though obviously none of the lists involved was meant to squeeze the maximum points out of the PL. Effectively, expect a 10% to 20% point difference when playing equal PL.
Another note, while Point costs are updated in Chapter Approved, power levels are NOT. Extreme example, the Reaver Titan is 120 PL. It was 2400 points originally. The 2017 Chapter Approved jacked that the points value up by 1600 to 4000... but it's still 120PL. The adjustments are unlikely to be as pronounced for normal infantry units, but keep it in mind.