To begin with, in order to burn at 16x, you need to have a pretty fast system and at least 512MB of RAM. I tried with 256MB and I got coasters with my 4163B at 16x. The same discs (Original Ricoh 16x DVD+R RICOHJPN-R03) burned fine at 16x with 512MB of RAM.
Next, Memorex does not make it's own discs. It gets other companies to make discs for them and you can never tell who will be making discs for them at a certain point of time. Since discs made by some companies work better than others for most burners, how can you tell what the quality of Memorex 16x discs will be from one batch to another if different batches are made by different companies?
Have a look at these posts for the various made Memorex discs in the market :
Here is someone else having problems getting Memorex 16x DVD+R to burn at 16x :
If the MID of your Memorex discs is RICOHJPNR03, then you should probably also know that the quality of those seems to vary quite a bit. I have used discs with these MIDs myself (original Ricoh 16x DVD+R, not a rebrand) and while they can burn at 16x, the burn quality is really not that good.
Here is more information on the variable quality of discs of this media code :
If you want to get decent 16x burns, get some decent 16x media like Verbatim, Taiyo Yuden or even Sony (I've used Sony 16 DVD+Rs and they are decent on a 4163B).