Any runway extension is welcome, however don't think that it solves all the problems.
For rwy 26 it would be fine, but rwy 08 would still be limited due to the hill by the junction to Ottery on the A30 (climb out gradient restrictions, etc). Operating A320's out of EXT in the winter to Tenerife, if the runway in use was 08 and we had a full load, we were close to the limits. It would be worse in summer; I never did it so I can't say how bad it would be, but I'd guess it would be significant on a really hot day. Extending at the 26 threshold doesnt help therefore and extending west from the present 08 threshold would be difficult (and expensive) due to the terrain and the small matter of Clyst Honiton.
On most days, big aircraft will opt for 26 with a tailwind. However if the tailwind component is above 10 kts, most (if not all) aircraft will have to use 08 with the corresponding reduction in take-off weight and hence range.