Linq has more overhead than queryspec, which overhead is minimal. You don't have to switch to a different query system however if you don't want to, the low level queries will keep on working with newer versions. Not having to migrate these could save you a lot of time. If you want to migrate to a newer query system, in general use what fits you best, where QuerySpec is recommended as it's the most flexible and closest to SQL.
To see the overhead of the query systems, see the non-change tracking individual fetch benchmarks of the different APIs in our benchmarks here: https://github.com/FransBouma/RawDataAccessBencher/blob/master/Results/2016-11-22.txt
There you can see that queryspec has a small overhead compared to raw SQL, but not as much as Linq. You can mix all query APIs in your application, you don't have to stick to one. If one query is easier formulated in Linq, and the other in queryspec, use both