The English translation of the databook says that Ichigo is fast like-lightning. 
However, it was not accepted for been unquantifiable. I understand that much.
But that was an error translation.
The raw Japanese databook says, "that Ichigo is fast as lightning" 
Which means Ichigo is Mach 289 in SS Arc onwards. Lieutenants maybe Mach 29 according to the previous calc, but not captains as shown by most of the elders that never train after the timeskip, but that is another discussion for another thread all together.
Can Gin Mach 500 debunks this? No.
Even if Gin didn't lie to Ichigo about his Bankai speed, it was still Mach 500. Ichigo couldn't had match him with Mach 289 as low-ball.
Hmm, I thought lightning would be faster than Mach 289. Though I'm not sure about this.
Also, you could ask on the Translations Request thread to see if someone would have the time to help out.
What inconsistencies do you mean? If you're referring to them suddenly jumping to Massively Hypersonic+ without any training after the timeskip, it doesn't really matter as that's how fiction works in general.
Now on the one hand, I'm not sure if a mere statement in a data book such as "fast as lightning" is enough to warrant an upgrade. While it is true that it is not as flowery and hyperbolic to the extent that statements in the Naruto data books and Fairy Tail editor statements are, it still might not be hard enough evidence.
But on the other hand, it does make sense considering the MHS to MHS+ ratings for post-time-skip characters, including those who are far out of their physical prime (and thus should not have improved) and at least two who might actually be weaker.
At this point is the only evidence that will fix the inconsistency speed calcs in the Bleach pre-time skip.
I'm doing more research on the new untranslated databook. It appears to confirmed that Gin lied to Aizen and not Ichigo. It also confirmed (if it's a correct translation indeed) that Gin Bankai is in fact Mach 500 (What he told Ichigo was true) and that the contraction/lenght speed of one of the techniques he use on Ichigo doubles that speed which would put Gin at Mach 1000.
This will be for another thread, but here is the evidence: , 
I'm with Soldier Blue on his first point. Though, it does make sense if the characters are MHS before the timeskip since almost all of them are MHS+ after the timeskip, the statement might not be enough especially since it comes from a databook. Regardless, I'm also fine with them remaining at High Hypersonic Pre Timeskip because there have been comparable or greater jumps in AP or speed in other series.
So, if it's accepted, Shikai Ichigo would be Mach 289 and (Hollowified) Bankai Ichigo would be Mach 500-1000, right?
But since LordAizenSama has retired from his position, who will decide on this issue?
There is no other word more accurate than "as" when compare to "like". Saying the word "as" is vague, is nothing more than a fallacy. But of course there should be a more accurate word according to your statement. Care to share that word with us?
I also disagree. With all due respect, the "speed of lightning", "lightning-speed", and "as fast as lightning" are all flowery language that gets tossed around without actually meaning the speed of lightning.
Very rarely do these phrases mean exactly what they say they do.