Vedad Ibisevic, scored 18 in the first half of the season in the 2008/09 BL season and never managed to replicate that form later in his career (though he didn't play the rest of that season because of an ACL injury, so one could argue that derailed his career).
Papiss Cisse, 13 goals in 14 games when he first came to Newcastle, we all know what happened after.
Generally, I would prefer we don't judge players based on 6 month purple patches, especially when their previous career doesn't show any similar performance. In Europe we've had recent 'one season wonders' who then haven't shown similar results afterwards, within the BL you also have Jovic and Kolo Muani with similar great single seasons leading to big transfers.
One difference with Guirassy is the low risk in terms of price. 17.5m EUR is pennies in this day and age, especially within the EPL, and I don't think he'll command an insane wage as well. In that sense, one could say it's worth a punt considering the guy is in insane form right now. One further issue though is that he'll be at the AFCON in January I think.