I suppose there could be lots of reasons for wanting a non-turbocharged IV - one would be cost.  If one were to build the plane simple and light you could end up with an honest 2,000-lb empty weight, at least 300 pounds lighter than many IVP's.  Then if you restrict the gross weight similarly - and extend the wingspan a bit with the winglets I would bet that the runway performance would be equivalent to most IVP's.  I think there are a lot of straight-wing IVP's flying at a gross of over 3400 - I estimate you will have better takeoff performance than them.  And the speeds at 10,000 ft or lower might be just as good or better than a IVP.  Sure, the high altitude climb gradient won't be as good, but a lot of people do almost all their flying below 12,000 feet.  In summary, while there is no such thing as too much power (or is there?), reducing the sea level power from 350 to 310 doesn't get it into the "not enough power" realm - if the weight is kept to a reasonable level.