Is it possible to upgrade Oct 2013 MBP-r SSD how much before it slows down?

Question

Hi all,
I was just wondering if its possible for Apple to upgrade the SSD in my macbook pro retina oct 2013 model,
I sorely wish that I had gone for the 256GB model really, I purchased applecare, so I don't want to void my warranty - besides, googling around I found out that there are no aftermarket upgrades available for it anyways.
I have set time machine backup to backup manually (is this okay?, I Don't tend to make mistakes of emptying unwanted stuff in the trash, or is it likely ill need to use it for more than this?)
Also, coming from PC's ive always noticed a considerable drop in performance once a drive starts getting alot of data, how much roughly does it take to slow down an SSD, is it the same as a spinny drive? Im still pretty new to OS X, first mac and first SSD and im panicking ive made a serious mistake purchasing such a small drive.
Thanks alot for your patience, I did have a search but im not brilliant at searching lol
Alex

Answer 1

Information for offloading data onto medium-speed and fast external disks:
Standard disks on Thunderbolt connectors are a waste of money (limited by the rotating disk speed at ~100 MB/s).  Standard disks on USB 3.0 run just as fast as on Thunderbolt.  You can use an external Seagate 1 TB hybrid SSHD to run at the maximum USB 3.0 and Thunderbolt speeds when accessing the 8 GB SSD cache.
Performance and price summary for Macs with USB 3.0 and Thunderbolt connections:
Expensive, Internal SSD,  Many 100s of MB/s,  120 to 1 TB GB capacity
~$100,  Standard disks on USB 3.0,  ~100 MB/s,  1 to 3 TB capacity
(Avoid 4 TB disks.  They are not as reliable.)
~$200 and up,  Standard disks on Thunderbolt,  ~100 MB/s,  1 to 3 TB capacity
(A waste of money.)
~$130,  Hybrid SSHD on USB 3.0,  ~100 to 200 MB/s,  1 TB capacity
($110 SSHD + $20 USB 3.0 enclosure)
~$210,  Hybrid SSHD on Thunderbolt,  ~100 to 300 MB/s,  1 TB capacity
($110 SSHD + $100 Thunderbolt enclosure)
"Seagate Backup Plus Portable Thunderbolt Adapter “
Expensive, External SSD,  ~350 MB/s,  120 to 1 TB GB capacity
R/W disk speeds:
USB 3.0 and Thunderbolt about equal at ~110 MB/s
Firewire about 65% at ~60 to ~70 MB/s
USB 2.0 about a third at ~35 to ~40 MB/s
R/W SSD speeds:
Thunderbolt fastest at ~355 to ~370 MB/s
USB 3.0 second at ~165 to ~195 MB/s
Firewire third at ~60 to ~70 MB/s
USB 2.0 fourth at ~40 MB/s
http://www.macworld.com/article/2039427/how-fast-is-usb-3-0-really-.html
http://www.storagereview.com/thunderbolt_storage_with_any_hard_drive_or_ssd