As has been pointed out by various posters here, it is definitely possibe to construct a tube frame chassis in such a way that it offers essentially equivalent safety to a carbon tub car. Indeed, the FIA has recently established tube frame standards which go a long way towards meeting this goal. These standard require real front and rear impact attenuation, a head surround, as well as composite side intrusion panels.
But.....are any current SCCA formula cars constructed in this specific manner? I don't think so. Everyone has a different perception of safety, but in my opinion a Swift 014a Atlantic tub (for example) is definitely safer than any FV,FF,FC,FB,FM, etc. car currently racing in SCCA. If you go to an SCCA event, most or all of the formula cars present will be older tube frame designs. I do not accept the argument that these older cars are as safe as the more modern carbon fiber Ralt or Swift chassis's. The tube frames probably handle front and rear impacts pretty well, but they simply do not offer the same level of side impact or of intrusion protection as a carbon monocoque.
Within the SCCA world, carbon tubs will in fact offer real safey advantages until we change the specifications of our tube frame chassis cars. Changing these standards will add weight to the cars. Could anyone build a 1000 lb FB car to the current FIA specs? I don't know.....it would be interesting to find out.