Introspectively I always felt that music helps me get into a focused state but I always wondered whether it has any effects. Over the course of May 2014 I collected some data on my own writing performance in different circumstances when I had a lot of written work to complete (a bit over100 hours spent on writing in that month).

Every 30min I took a break and gave a 1-10 rating of the quality of the work I had completed in that period, and brief notes about anything else that might be notable. I admit that self-rating is rather arbitrary but simply word count wouldn't suffice as I was also editing, consulting sources and other tasks related to writing at various times. And of course these results may not generalise to anyone besides myself (and indeed for my own purposes I should do a replication next time I have a huge writing crunch). Mean self-rating of "quality":

* Office, no music: ___ 5.4 (10.5hr)
* Office, instrumental: 5.8 (21.5hr)
* Office, vocal: ______ 5.9 (20hr)
* Library, no music ___ 4.2 (5.5hr)
* Home, instrumental __ 5.3 (23hr)
* Home, vocal _________ 5.5 (21.5hr)

The mean ratings above conceal a lot of variability; the only reliable effect (Wilcoxon-Mann-Whitney test) was that the university library is a horrible place for me to get anything done (No music: Library < Home and Library < Office). No surprise there - as in my undergraduate studies the library seems mostly to be a place people go to avoid doing work. The apparently lower mean in "Office, no music" was driven by a couple of outliers related to distraction by other people.

main musical styles (not possible to analyse due to variability): old thrash metal, new doom metal, psychedelic folk music, rockabilly, bluegrass, shoegaze, bebop/hard bop, J-pop, person with guitar.

final note: I do not tend to notice the details of lyrics unless i am paying very close attention to the music, even for highly lyrical music I still mostly focus on the instrumental parts.

A quick search in those references for "lyric" turns up only this paper from 2011: A Functional MRI Study of Happy and Sad Emotions in Music with and without Lyrics

Abstract:

Musical emotions, such as happiness and sadness, have been investigated using instrumental music devoid of linguistic content. However, pop and rock, the most common musical genres, utilize lyrics for conveying emotions. Using participants' self-selected musical excerpts, we studied their behavior and brain responses to elucidate how lyrics interact with musical emotion processing, as reflected by emotion recognition and activation of limbic areas involved in affective experience. We extracted samples from subjects' selections of sad and happy pieces and sorted them according to the presence of lyrics. Acoustic feature analysis showed that music with lyrics differed from music without lyrics in spectral centroid, a feature related to perceptual brightness, whereas sad music with lyrics did not diverge from happy music without lyrics, indicating the role of other factors in emotion classification. Behavioral ratings revealed that happy music without lyrics induced stronger positive emotions than happy music with lyrics. We also acquired functional magnetic resonance imaging data while subjects performed affective tasks regarding the music. First, using ecological and acoustically variable stimuli, we broadened previous findings about the brain processing of musical emotions and of songs versus instrumental music. Additionally, contrasts between sad music with versus without lyrics recruited the parahippocampal gyrus, the amygdala, the claustrum, the putamen, the precentral gyrus, the medial and inferior frontal gyri (including Broca's area), and the auditory cortex, while the reverse contrast produced no activations. Happy music without lyrics activated structures of the limbic system and the right pars opercularis of the inferior frontal gyrus, whereas auditory regions alone responded to happy music with lyrics. These findings point to the role of acoustic cues for the experience of happiness in music and to the importance of lyrics for sad musical emotions.