According to the Torah, the mitzva of reading Shma is all night. There is a מחלוקת about the דין מדרבנן. Chachomim hold that you can only read until chatzos in order to make sure you read on time. Rabban Gamliel says that the proper time is until dawn [Mishna Brachos 2a].
There is a machlokes Rishonim as how to understand the the Mishna. Rabbeinu Yonah [ברכות א א ד"ה ויש להקשות] says that according to Rabban Gamliel לכתחילה you should say Shma until chatzos and if you didn't then you can say it afterwards. According to the Chachomim - after chatzos is not an option. According to the Rambam [הל' קריאת א-ט] the Chachomim hold that you may read after chatzos בדיעבד while according to R"G one can read after chatzos even לכתחילה. The Shulchan Aruch ['רל"ה ג] paskens that if one read after chatzos then he is yotzei [like the Chachomim according to the Rambam].
What would the דין be according to R"G [if he holds that one can only read after chatzos בדיעבד] if reading before chatzos would mean that he wouldn't properly enunciate the letters [מדקדק באותיותיה] (maybe b/c he is rushed) while if he reads after chatzos he would say it properly?
On one hand - He should read it properly after chatzos b/c that is a דין דאורייתא as we see from the gemara [ברכות ט"ו,ב] where we derive from "ולמדתם" that it should be למודך תם - he should make space in between words.
On the other hand - he should read before chatzos b/c according to the Chachomim if he reads after chatzos he is not yotzei at all [according to Rabbeinu Yonah].
We see from the gemara [.שבת ד] that if one put dough in the oven [as opposed to the bank - where it belongs!!] on Shabbos, the rabbis allowed him to remove it [an איסור מדרבנן] in order to avoid the איסור מדאורייתא of baking on Shabbos if he leaves it in the oven. So here also, maybe the rabbis allowed him to transgress the מדרבנן of reading until chatzos [לכתחילה according to R"G as per the understanding of Rabbeinu Yonah and according to the Chachomim in the understanding of the Rambam] in order to fulfill the מדאורייתא of reading with proper enunciation.
Or one can argue that one is permitted to transgress a מדרבנן in order to avoid a לאו [such as baking on Shabbos] but not in order to fulfill an עשה [such as clearly reading the words of Shma]. We see for example that if one's lulav or shofar was stuck in a tree, Chachomim forbade one from transgressing the דרבנן and taking it down even though that would mean that he would be prevented from fulfilling the דאורייתא. So we see that Chachomim insisted we follow their Halacha even at the expense of a דאורייתא.
[מאוצרות הג"ר אברהם גנחובסקי זצ"ל]
