I agree there is some doubt and confusion about what "open up 2 stops" and variations to that effect. Many years ago, we visual arts students were perplexed as to just what this meant, looking for an "opening" and "stops"!! We were expected to known even though it wasn't explained in details. Published tables (i.e. Hoya's) are often not much more help to those seeking guidance, often giving just filter factors and the stops difference. Ideally, such references should include '+' (much less commonly '–') as the equivalent compensation to the stated EV (FF) factor.
Unless I am in a hurry (i.e. quite often I'm nigglingly tolerating getting very wet, on a slippery perch or some other environmental or technical hazard), the POL will be in-situ on the lens and metered 'there and then' (the onboard meter is deadly accurate); I can always go back and fine-tune metering once I review the results, whichever method of metering I employ. A tiny 30mm diameter POL (costing all of AUD$73 about 18 months ago!) is swapped on my L758 spot/incident meter's receiving lens.
Regarding how visible +0.5 stop is on reversal film, it will be very visible (just as –0.5 will be), often enough to balance the affect of the POL with a pleasing photograph. So too, will +0.6 (2/3), as that is greater than +0.5, of course. I think you should willingly burn a few rolls of film in the name of experience, exposing with a POL to gauge the affect it has. Cut the talk, go out now and shoot, damn it.