Is there any difference betweeen a giyur m'safek and a giyur l'chumra? In either scenario is a female who undergoes either of these conversions prohibited from marrying a kohein?
One diffrence would be if we have doubts about the conversion such as if we don't know if the mikva the person immersed in had the proper amount of water. This would make the person a safek jew and require them to dip again, a giur misafek. Wheres a situation where there is an argument concerning a certain law of conversion and we try to be stringent and the person converted according to the lenient opinion and we wanted them to convert again according to the stringent opinion, that would be a giur lichumra. But in all probability whatever case you are talking about is probably an interchangeable term as Yishai said.
The terms are used interchangeably (see e.g. here). So you can't determine anything from the usage of the term.
In terms of prohibition to marry a Kohen, that will depend on the individual circumstance which required the conversion. Sometimes it is recommended out of an abundance of caution (e.g.), but that would not invalidate marrying a Kohen. On the other hand, if the doubt were greater it may.
So it is really a case-by-case thing, and no conclusion can be drawn from the specific phrase used to describe it.