diff --git a/README.md b/README.md
index 60d0760f..7cdbbdb9 100644
--- a/README.md
+++ b/README.md
@@ -23,7 +23,7 @@ These are configurations on which lowRISC is focusing for performance evaluation
 | Config | "small" | "maxperf" | "maxperf-pmp-bmfull" |
 | ------ | ------- | --------- | ---------------- |
 | Features | RV32IMC, 3 cycle mult | RV32IMC, 1 cycle mult, Branch target ALU, Writeback stage | RV32IMCB, 1 cycle mult, Branch target ALU, Writeback stage, 16 PMP regions |
-| Performance (CoreMark/MHz) | 2.44 | 3.09 | 3.09 |
+| Performance (CoreMark/MHz) | 2.47 | 3.13 | 3.05 |
 | Area - Yosys (kGE) | 33.15 | 39.03 | 63.32 |
 | Area - Commercial (estimated kGE) | ~27 | ~31 | ~50 |
 | Verification status | Green | Amber | Amber |
@@ -31,7 +31,9 @@ These are configurations on which lowRISC is focusing for performance evaluation
 Notes:
 
 * Performance numbers are based on CoreMark running on the Ibex Simple System [platform](examples/simple_system/README.md).
-  Note that CoreMark was compiled without support for the B extension.
+  Note that different ISAs (use of B and C extensions) give the best results for different configurations.
+  See the [Benchmarks README](examples/sw/benchmarks/README.md) for more information.
+  The "maxperf-pmp-bmfull" configuration sets a `SpecBranch` parameter in `ibex_core.sv`; this helps timing but has a small negative performance impact.
 * Yosys synthesis area numbers are based on the Ibex basic synthesis [flow](syn/README.md).
 * Commercial synthesis area numbers are a rough estimate of what might be achievable with a commercial synthesis flow and technology library.
 * Verification status is a rough guide to the overall maturity of a particular configuration.
diff --git a/examples/sw/benchmarks/README.md b/examples/sw/benchmarks/README.md
index 78c2a9ec..1ed303cc 100644
--- a/examples/sw/benchmarks/README.md
+++ b/examples/sw/benchmarks/README.md
@@ -62,6 +62,32 @@ seedcrc          : 0xe9f5
 Correct operation validated. See README.md for run and reporting rules.
 ```
 
+### Choice of ISA string
+
+Different ISAs (to choose different RISC-V ISA extensions) can be selected by
+passing the desired ISA string into `RV_ISA` when invoking make.
+
+```
+make -C ./examples/sw/bencharmsk/coremark clean
+make -C ./examples/sw/benchmarks/coremark RV_ISA=rv32imc
+```
+
+This will build CoreMark using the 'C' extension (compressed instructions).
+
+When changing `RV_ISA`, you must clean out any old build with `make clean` and
+rebuild.
+
+The following ISA strings give the best performance for the Ibex configurations
+listed in the README:
+
+| Config               | Best ISA |
+|----------------------|----------|
+| "small"              | rv32im   |
+| "maxperf"            | rv32im   |
+| "maxperf-pmp-bmfull" | rv32imcb |
+
+### CoreMark score
+
 A CoreMark score is given as the number of iterations executed per second. The
 CoreMark binary is hard-coded to execute 10 iterations (see
 `examples/sw/benchmarks/coremark/Makefile` if you wish to alter this).  To obtain