g maps doesn't use true realtime 3D graphics. The Tom Tom is basically 2D technology made to look 3D through perspective. The level of graphic detail is much different. Navigation renders the entire map in 3D with 3D buildings. This takes more power. Not to mention g maps uses far more battery too
Memory leaks are not limited to the OS level either. Plus Apple has been notorious to not have fully optimized new versions for older devices until a .1 release. There's gonna be bugs plaguing the system on new OS versions. This is the same for other systems too (vista anyone??)
There was a person who worked on Microsoft Vista on either MacRumors or during the Reddit AMA that mentioned the reason Vista was "bad" at the time was due to people still using computers utilizing computers with Pentium 4 and less than 1 GB of RAM which Vista was not made for. Most computers now use something comparable to Core Duo 2 with at least 2 GB of RAM so if Vista was released today it wouldn't have such the back lash as it does today. Simply Vista was too far ahead of its time. (though this statement can be debatable even when Vista was announced Pentium 4 was way far too old to even be properly supported it was released in 2000 when Vista was released in 2007)