external burner write faster at 8x than 16x?

The same data on the same type of disc? thats very odd.

Thats part of the YYxtimes myth.
Most 8x burns are done with 8x speed through all of the disk.
16x burns typically start at 8x and get to higher speeds (either continually or in steps) and only reach 16x at the outer parts of the dvd, so if the dvd is not full you won't even reach 16x, not even for a short period.

And as second limitation most external USB2.0 and firewire enclosures are not capable of sustaining transfarerates high enough to satisfy 16x needs (and most even have problems sustaining constant 12x).
This depends largely on the used USB->IDE Bridgechip and the dvd-drive that is used, not to forget the USB-Chip used in your computer and the drivers and OS used on it. Not every bridgechip works well with every drive.

Now let's have a look on what happens if you try to burn with 16x and your external enclosure/drive combo is not able to sustain 16x speed (let's say 10x is the speed it could manage):
-You start to burn and start with 8x, no problem, same speed as with 8x burnsetting
- Now you get to the point where you switch to next speedlevel. First, this takes a small amount of time as the drive needs to speed up and the burner has to wait for that, so you already loose 2 seconds (just a guess). Now you continue to burn at 10x without problems - hey, you gain time on the 8x burn.
-Now your burner switches to 12x, you again loose some seconds for speedadjustment and now you have a problem, your drive does not get the data fast enough from your computer so now you get into the situation where you some years ago would have met a "buffer underrun", but today techniques like justlink kick in and stop burning for a few seconds until buffers are again full enough and when they are, burning starts again with 12x - shit, again too fast, waittime again.
Guess what happens when the drive switches to 14x and 16x burnspeed.

So you end up loosing time everytime the drive has to wait for data and end up at not gaining much or even loosing time (depending on the stustained transfarerate your external drive is able to achieve) when switching from 8x to 16x burnspeed.

Sorry for my poor english, but I hope you got the point - just don't trust the numbers the industrie want's to sell to you (my beloved example are the GB numbers on harddrives which steal ~5% from you).

As the datadensity on the dvd is the same on all of the medium you need more rotationalspeed to read the same amount of data in the same period of time when you read the "inner" part of the dvd compared to the outer part.

As you can't increase rotational speed as far as you like (as you always have slight weight differences on the disks which lead to "unbalanced rotations" - sorry, no idea what "unwucht" might be translated to).
So - last time I checked specs - the dvd-drives rotate the dvds at 10.000 rpm max (which is the reason you get to about 8x at starting time).
When you travel from the inner part of the dvd to the outer part without reducing the rpms you will get to faster speeds.
To "stay" at a constant writingspeed the drive has to slow down continually to keep the speed and when you than get to the next speedstepping the drive rapidely needs to spin up to full (or almost full) speed again and than starts to burn with the next speedsetting.