I will try answering although I lack refrences. Pronunciation of a mantra is the most important. Consider the lines itself from Ganpati Atharveshrisha-
"गणादिं पूर्वमुच्चार्य वर्णादिं तदनंतरं।।
अनुस्वार: परतर:।। अर्धेन्दुलसितं।।
तारेण ऋद्धं।। एतत्तव मनुस्वरूपं।।
गकार: पूर्व रूपं अकारो मध्यरूपं।
अनुस्वारश्चान्त्य रूपं।। बिन्दुरूत्तर रूपं।।
नाद: संधानं।। संहिता संधि: सैषा गणेश विद्या।।
गणक ऋषि: निचृद्रायत्रीछंद:।। गणपति देवता।।
ॐ गं गणपतये नम:।।7।।"
I want to specifically cast attention to the last 3 lines. Where its stated "बिन्दुरूत्तर रूपं।। नाद: संधानं।". The words equipped with "Nada" are mantras. Not only pronunciation, the mantra consists of specific amount of alphabets, the Nada affects the Nada Bindu. The sound beigns from the Bindu and ends there as well. Another example is "Naysā" in which diffrent body parts are touched and specific matras are chanted. Like "Karanyasa", "murtinyasa" etc. The Matras runs over a perticular varga like 'ka' varga. This poetic form of mantra is itself what makes mantras effective.
Wrong chanting of mantras is often done, to make the chanting correct practice is must. How effective is a wrongly chanted, mis-pronoused shloka? It's not answerable. On what basis the comparison is to be made? I also could not found any references on that. But here is some information on Nada and Nada bindu.