Guardrail metric is just another metric, nothing special, you are just making sure it's not moving into unwanted direction. You can apply the same calculation for Type 1 and 2 errors as to any other metric when doing power analysis. If you don't want to dive into all the complexity related to ratio metrics, you can just compare two data sets after calculating averages for each user. In some cases it's not the right thing to do, but in your case I think it would be alright, as you are probably interested in avoiding rather big move of the guardrail metric. So it's just as simple/complicated as getting power analysis for two numeric datasets (variations) of page load times (average per user) using any statsig tool.