Eastern tech is +20% tech cost. Poland can easily get Western Arms Trade (ally an independent neighbouring Western country and maintain 150+ relations), which gives -10% tech cost (and goes away when you westernize). That means that every tech you buy as Poland, before considering NIs, idea groups, neighbour bonuses, etc. is only 60 points more expensive than the same tech bought by a Western tech country.
Westernizing as an Eastern tech country costs 2400 monarch power across the three categories. Assuming for the sake of analysis an even 800/800/800 split, that means it takes (800 / 60) = 13.33 tech intervals (i.e. on the order of 170-190 years) for the process of Westernizing as Poland to pay off at the crudest and most naïve level of analysis.
But! A monarch point is more valuable the sooner you get it, because the sooner you have it, the sooner you can spend it and thus the longer the time for which you can reap the rewards of spending it.
Thus: No, Poland (or, really, any Catholic Eastern-tech country) should not Westernize, and it's at least somewhat questionable for the Orthodox members of the Eastern group.