There are a couple of different ways to answer your first question. I will attempt an answer from a linguistics perspective, specifically with regards to the lexical aspect of the verb in question.
The dominant perspective on lexical aspect of verb tenses for the last few decades has been Actionsart. This deals with how the verb interacts with time. Specifically, there is a sense of anchoring the verb to a point in time, usually the point in time in reference to the author. Broadly speaking the loose categories for tenses under Aktionsart are as follows:
- Imperfect: Past action with incomplete results
- Perfect: Past action that has led to the current results
- Present: Current action
- Future: Future action
- Pluperfect: Action that occurred prior to the time-frame being considered
- Aorist: Generic/"default" tense that describes a nondescript period of time
There are, of course, nuances within these (e.g. an iterative present) that get to an absurd level of granularity. This is due to exceptions that violate a hard-and-fast rule of how the verb tense interacts with time.
An emerging perspective in Hellenistic Greek studies is that of Verbal Aspect Theory, most notably championed by Stanley Porter. Within this stream of study, verb tenses are less anchored in time and language is viewed as a tool that the author uses to communicate a message to their audience. The aorist tense is still perceived as a default tense, but any variation from that tense would cause the initial reader/hearer to notice the shift from what was default. Which tense the author chose to use would, in turn, elicit a different sort of reaction from their audience.
Both paths endorse a sense of "markedness" which is supported by the relative rarity of the tense in question. Pluperfect is the highest-marked tense as it is very rare. Perfect is also highly-marked as it is also quite rare. Under Aktionsart, this tense would cause the student to evaluate how it related to the time-frames in question, and would also guide the specific translation of the word/phrase/clause/pericope. Under Verbal Aspect, the student would evaluate the phrase/clause with the highly-marked verb as "foregrounded" over against the surrounding content.
How does this relate to this passage?
ειρηκεν, as you have noted, is a perfect (past action -> present state), active, indicative (mood of reality), third person. The primary takeaway from this would be that τὸν κύριον (the referent of the third person verb in question) said something to Paul and the state of that reality is still true and has led to his current state of understanding. Woodenly this could be translated as "has said."
ειρηκεν, as you have noted, may represent a shift in the expected. However, in this pericope (vv1-10) the breakdown of tenses is as follows (SBLGNT):
- Aorist 11
- Future 6
- Perfect 8
- Present 15
Normally we'd expect to see Aorist overwhelmingly dominate the tenses in any given pericope. Present would be the next most common, but it is the most common in this passage. This isn't entirely abnormal, but is noteworthy, along with the heavy use of Future and Perfect tenses. Additionally, looking at the flow surrounding this verb we see Presents and Aorists scattered throughout. What does all of this mean? In order to see the importance we should look at the remainder of the chapter. Here's the breakdown on vv.11-21:
- Aorist 22
- Future 3
- Imperfect 1
- Perfect 3
- Present 17
This stands in stark contrast to our passage which, on the balance, seems to have an abundance of more highly-marked verb tenses. This passage, on the other hand, has a more standard distribution of tenses. So back to the "so what?" question. Vv.1-10 constitute a section that Paul wishes to highlight for his audience. Given the context of the letter, Paul is accentuating his boast in what the Lord has told him. Paul is using nonstandard tenses to ensure that this content is foregrounded against much of the surrounding content.