I wouldn't worry about what gear you are in alone as it needs to be considered in conjunction with what speed, rpm and what ability the car has to accelerate FREELY from the point of activation.
For example;
If you were on a downhill slope, in top gear, accelerating up to say 4,000 rpm (at whatever speed) and then activated the nitrous, it should be fine.
In the other extreme; if you were on an uphill slope, struggling to pull say 2,000 rpm (at whatever speed) and then activated the nitrous, you'd probably break a piston.
The trick is to avoid the negative side of the point at which these two examples merge.
One thing I would suggest you do, is call the office and tell them what size jets you are running, to make sure that you have a suitably large enough jet to match the higher nitrous flow from having a heater. As Alex stated above, adding a heater equates to having fitted a bigger nitrous jet and in your example of pressure changes, that's an increase in the region of 30%, which means that you are now running ROUGHLY 30% leaner than before.
When we sell kits without heaters, we try to take into account the time of year they are being bought and will probably be used in, which in the UK is usually well down from the max that can be seen on hot days (as you 700 psi proves). We do that by supplying the kits with fuel jets that are one or two sizes down from what we'd run on a hot day or with a heater.
When running with a heater on an average day a ratio of 2:1 is usually acceptable but for the 4 following reasons, I'd suggest you go one more jet size bigger than that (on the fuel), if I were you;
1) Your motor is relatively small
2) As far as I'm aware it still has the original cast pistons
3) It sounds like you're using it quite often compared to say a Drag racer
4) If you intend to use it over the next few cooler months, the NA oxygen level will be higher and although your ECU should correct for that, it may not be able to do so when the nitrous is also flowing and pulling more NA oxygen in.
Increasing the fuel jet can't do any harm and could not only make the nitrous use safer/kinder to the motor but also produce even better performance results.
