Is Cedric still eating any dry food, or his he on a completely wet diet now? Was dry food removed recently, or before he started insulin?
It looks like one of two things are happening here. First, when a cat has had consistently high BG for a while, once he gets down to normal numbers the cat's liver panics and thinks that his BG is too low, and dumps glucose into his blood. We call this "bouncing." Eventually his liver will figure out that the normal BG is safe, and stop the bounce, but sometimes it can take a while. Bouncing is absolutely normal for a newly diabetic cat, and dose shouldn't be raised based off of a bounce.
What is really concerning me, however, are those low preshot numbers. I would need more data to know for sure, but I believe the insulin dose is too high. Most cats on a wet, low-carb diet don't need much more than 1u of Lantus. Similar to bouncing, when a cat gets too much insulin glucose gets dumped into the blood as a survival mechanism--otherwise the cat would go hypo. If he's already going low, then 2.5u is way too much insulin, and dangerous. I'm very concerned that your vet raised the dose to 2.5u after seeing those low pre-shot numbers, because the dose should have probably been lowered. Also, Lantus dosing needs to be consistent, on a strict 12/12 schedule. There's room to wiggle with other insulins, but not Lantus or Levemir. A dose needs to be held for at least 3 days to see how it's effecting BG. At least three numbers need to be collected daily to determine the right dose--the AMPS, PMPS, and one or more numbers about halfway between the shots, in either the AM or PM cycle.
I would urge you to read the dosing protocol for Lantus, and possibly print it out to give your vet (that's what I did with mine!)--it's been proven to safely bring hundreds of cats (including my own) into remission:
http://www.felinediabetes.com/FDMB/viewtopic.php?f=9&t=1581
I would also urge to set up a spreadsheet to help keep track of your data. This way you can ask advice of the dosing experts in the Lantus forum. They've helped many, many kitties on Lantus to regulation and remission!
http://felinediabetes.com/FDMB/viewtopic.php?f=6&t=18207&start=0
And there's a wealth of information in the Lantus forum that will help you--the stickies are lifesavers! (literally.)
http://www.felinediabetes.com/FDMB/viewforum.php?f=9