Two things cause wear on elements- high temperature and cycling. I did an informal test a few years ago when I was firing cone 8. Instead of firing all the way to cone 8, I started firing to cone 6 and holding for 40 minutes to put down cone 8. My elements were cycling a lot more in order to hold temp, but they were never firing as hot. The result was that my elements lasted about 25% longer, possibly more (as I said it was an informal test). So higher temps seemed to negatively affect element life more than switching.
When firing fast, the elements spend more time 'on' than when firing slow, switching less. Because they run slightly hotter than the interior temperature of the kiln, they are, in essence, spending more time at a higher temperature than if you were firing slow. That could mean decreased element life. But even in a slow firing the elements spend a great deal of time 'on' when the kiln gets to the top half of a firing. If you had data logging capabilities you could test how often the elements cycle at the top half of the firing and compare slow to fast.
The slip is not vitrified and the glazes, while delaying seepage, do not compensate for the lack of vitrification. Standard shows an absorbency rate of 5% for the terra cotta slip. That is still rather porous at cone 04. For water-tight, you need to get to 2% or less for absorbency -- preferably in the 1% range -- that is my comfort level for vases and other forms holding water for long periods.
Are you just using a liner glaze? or glazing both interior/exterior? You could try bisque firing at higher temperature -- maybe consult with Standard to see how far you could push the slip body. Some folks working in terra cotta apply terra sig to the bottoms of pots to help seal the clay body; again, that can delay seepage but it will not compensate for an unvitrified clay body.