There's no need to estimate the Haswell performance... there are plenty of Haswell CPUs in Macs already that we can compare with. Now of course it's possible they may have refreshed the Mini with mobile Haswell CPUs instead of desktop Haswell CPUs but if they had chosen to go the same route as the iMac, this is what the results would look like...
The nMP Quad scores around 3500-3600 on single core, and 13,000-14,000 on multi-core. And there is no debate that the high-end BTO iMac can better these scores.
So if a Haswell Mac Mini offered the same BTO quad core as the iMac (i5-4771) it would score between 3800 and 4000 on Geekbench in Single Core and 14,000 to 15,000 on Multi core.
If a Haswell Mac Mini offered the same base quad core as the iMac (i5-4670) it would score between 3500 and 3900 on Single Core and 11,000 and 12,000 on multi-core.
So depending on the Haswell CPU used, a $1000 Mac Mini could have beat the entry level $3000 Mac Pro in both single core and multi core benchmarks, or at least kept up with it in Single Core.