<p>I realize this is a quite general question, but I am wondering about the performance impact of implementing business logic with WF contra imperative code in fairly large enterprise system. I would like to hear others experiences in this regard.</p>

You can check
Performance Characteristics of Windows Workflow Foundation
for key performance considerations and modeling guidelines that are important when developing applications on top of the Windows Workflow Foundation.
However I would suggest to wait for the WF 4.0 announcement at PDC (end of October) since
they are promising
Significant improvements in
  performance and scalability
(Ten-fold
  improvement in performance)
EDIT (after PDC, 11Nov08):
OK, not much revealed about performance at PDC, although Kenny Wolf
said in his talk
that there are 10-100X performance improvements from execution speed to persistence sizes.
Moreover, Rick Garibay posted some post-talk discussions with Kenny Wolf
here
. I am copying a relevant passage:
My first question was around the
  impetus for such a major overhaul.
  Kenny shared that they focused very,
  very deeply on key customer requests
  and opportunities based on earlier
  versions of the framework. There were
  3 feature types that resonated most.
  First, is delivering a truly
  model-based framework that allows
  every aspect of the WF to be
  represented declaratively. Second, was
  being able to truly participate in the
  repository ecosystem, which is key for
  deployment, management and operations
  scenarios. And last, but not least was
  performance.
Earlier in his talk, Kenny cited
  intriguing improvements, including 10x
  to 100x performance improvements and
  persisted workflows becoming
  essentially "free" with WF 4.0. In our
  more one-on-one discussion after his
  talk, he used the analogy of swinging
  at a baseball with a wooden baseball
  bat. While the wooden baseball bat is
  effective, it feels a bit heavy, if
  not clunky at times. When you pick up
  an aluminum baseball bat, it is
  markedly lighter, and feels
  significantly more aerodynamic. WF 3.0
  and 3.5 work, but WF 4 is a new and
  improved aluminum baseball bat.