[Joe R Huff Sr]

Just wondering if anyone has experience with running the RW 162 engine above the 4250 for extended time,

I am looking for what would happen IF the engine was run at say 5000rpm, continuously would it make more power, would it explode, would it last?

In this idea I am not using the FADEC system I am using the TBI system which I have proven at the regular 4250 rpm, and my outside scoop cooling system.

But just thinking outside the box.... what if a gearbox was designed with an output at 520rpm but the input around the 5000 would the engine be OK?

I am under the assumption you are referring to the "stock" 162 engine.  If that is correct I will make a statement regarding that assumption.
If my memory serves me correctly the 162 has a 4.0" bore, 3.28+- stroke and a 5.4" rod.  The cylinder head configuration is the limiting factor in the desire to increase the RPM in order to produce more power.  At 4250 the engine is below 85% VE and atmospheric pressure at sea level.  The short rod increases the angular velocity to a point of loosing power above 4000 RPM.  Camshaft cannot make up the needed increase.  At 5000 RPM the valve springs would be consumed prior to 500 hours.
That being said, not to be a pessimist but realist, the engine is a poor design.  The naturally aspirated 162, fuel injected engine averaged 123 HP at 4250.  Injector angles, atomization, compression ratios, all were incorrect and I won't even talk about the valve geometry.  
I have developed a better engine for the helicopter. I have three different versions.  N/A 4250/160HP, N/A 4250 175HP and a turbo version 3900 225HP.  All of these require a complete change of internals including crank, rods, pistons, cam, and injectors.  I, along with help from Kyle Carter and Brad Champlin have made a very good 160HP n/a engine using the FADEC that was used in the earlier 162's.  
Sorry for such a long reply