FLHerne

You can certainly get faster than 300m/s with Wheesleys. This one does 700m/s in level flight, and can lob itself above 20km for those annoying high-altitude readings.

The efficiency (Isp) of NERVs is massively affected by atmospheric pressure - they're over 4x less efficient on the surface of Kerbin than in vacuum. The maximum fuel flow is constant, so the maximum thrust is reduced by the same factor. More generally, there's no point doing things you find stressful and unenjoyable. If you're tired of crashing landers into a variety of orbital bodies, leave them alone for a while and find something else to blow up. Spaceplanes are interestingly tricky with 1.0, for example, and have a much shorter turnover than interplanetary probes between launch and disintegration.