No more than 25% although 15% would seem likely clock for clock. They haven't really announced new features so it's hard to measure the level
Confirmed new features
Haswell New Instructions (includes Advanced Vector Extensions 2 (AVX2), gather, bit manipulation, and FMA3 support).
New sockets — LGA 1150 for desktops and rPGA947 & BGA1364 for the mobile market.
Intel Transactional Synchronization Extensions (TSX)
Graphics support in hardware for Direct3D 11.1 and OpenGL 4.0
DDR4 for the enterprise/server variant (Haswell-EX)
Variable Base clock (BClk) like LGA 2011.
Supervisor mode access prevention (SMAP)
Expected features
Shrink PCH from 65 nm to 32 nm.
A new cache design.
Up to 32MB Unified cache LLC (Last Level Cache).
Support for Thunderbolt technology.
There will be three versions of the integrated GPU: GT1, GT2, and GT3. According to vr-zone, the fastest version (GT3) will have 20 execution units (EU).Another source, SemiAccurate, however says that the GT3 will have 40 EUs with an accompanying 64MB cache on an interposer. An additional source, AnandTech, agrees that GT3 will have 40 EUs, and states there will be a version with up to 128MB of embedded DRAM, but makes no mention of an interposer. Haswell's predecessor, Ivy Bridge, has a maximum of 16 EUs.
New advanced power-saving system.
Fully integrated voltage regulator, thereby moving a component from the motherboard onto the CPU.
37, 47, 57W thermal design power (TDP) mobile processors.
35, 45, 65, 84, and ~100W+ (high-end, Haswell-E) TDP desktop processors.
10W TDP processors for the Ultrabook platform (multi-chip package like Westmere) leading to reduced heat which results in thinner as well as lighter Ultrabooks, but performance level will be lower than the 17W version.