In general, it would gain 1 die per boost level, and that extra boost is for free. I had a look at it, Guiding bolt is a better choice if you were going to choose between 2d12 TOTD boosted 1st tier compared to 1MP for 4d6 Ranged Attack which in your case your 15 DC is the attack vs their AC so +5. Against low AC creatures, Guiding bolt is better, against low WIS creatures TOTD is better. For close combat you have Inflict Wounds 3d10 un-boosted 1MP, this is also better than TOTD with 1 boost. Given TOTD has provisions to boost it further at level 5 without "boosting" it's fair to say it'll be a favorite go to choice until you need to worry about AOE. Spirit Guardians is not until 3rd tier, that's level 5, and by then you could have a 15ft radius nova of sorts, boost that with the 6MP focus for 4d6 on everyone within a 15ft radius, that's pretty neat, add the staff of goblins among other boosts and that's great but it does center on you. Wizards and Sorcerers have a feat that lets them project a spell an additional 30FT even spells meant to be close range.
So yes, Toll of the Dead is a good one for an already injured foe.
Let me now just say for spells that can be damage boosted with the focus, illegally, the chances of having a successful boost are very good. I would give negative consequences for rolling 1, 2, 3, and extra positive consequences for rolling 18, 19, 20, and a sliding scale in between depending on many things but generally performing as expected. Yulya won't go to wizard jail for illegal spellcasting, I said illegal because it's not possible with vanilla but I like the focus mechanic and with a huge focus boosting a spell up to level 9 just sounds fun if you don't mind the consequences for rolling low.
In case you think I'm suggesting Guiding Bolt, I'm not, it's not statistically better in this case considering his WIS is +3 and his AC is effectively +5 vs your DC15.