Wouldn't you expect Navi to come later, given how AMD still struggles to release Vega?
Do they actually? Beyond some questionable rumors from last year Vega was always pegged at 2017 in their roadmaps. Most people expected it in the 1st quarter but that was nothing more than wishful thinking, and as far as we know AMD might have always planned to launch Vega a year after Polaris.
There's also the fact that Vega based Radeon Instinct MI25 is available since January pointing to the fact that there's nothing wrong with Vega itself and the most obvious reason for its late arrival is the current prices of HBM2 modules.
Since Navi will most likely use HBM2 as well this shouldn't be an issue in 2018 and thus it's unlikely that Navi will be impacted by it in the same way Vega is. So I think it's pretty safe to assume that Navi is coming around the same time as Polaris and Vega did in the previous years.
I don't expect this to happen that early. The Ti cards have generally come out only when Nvidia has been able to get good enough yields for the almost full chip of whatever generation. They usually peddle a Titan version for early adopters then halve the price and sell a Ti that can match it in performance even though not in specs.
Titan Xv - Jan'18
2080Ti - Jun'18
That gives the new Titan ~6 months of reign which is very much in line with how it was previously.
Does memory speed even make that much of a difference? GDDR5X vs GDDR6 for example.
That's a good question as so far we don't know which speed GDDR6 will launch at. The promise of 16Gbps is just that - a promise. GDDR5X had it's speed promised to be 12-14Gbps and so far, almost a year later, it's only reached 11 (12 if we count some unstable OC in). So it's entirely possible that first GDDR6 cards won't be anywhere close to 16Gbps and will in fact launch at speeds closer to that of promised for GDDR5X - 12-14Gbps.
In this case the speed difference between GDDR5X and GDDR6 becomes pretty much irrelevant and thus it may well be that GV104 will launch with GDDR5X and will get a GDDR6 upgrade at some point next year for example.
GDDR6 isn't much different than GDDR5X, the gap won't be bridged.
GDDR6 should run on lower voltages and thus consume less power on the same speed as GDDR5X though.