What is another way of saying "we cannot guarantee"?
To the customer:
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All work will be performed according to your specifications.
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The hardware configuration that you requested has not been previously built and tested by us. We therefore cannot offer any estimates as to how well it will perform.
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If you wish to modify the hardware configuration after having confirmed that it meets your original specification, regular service charges will apply.
There is nothing to stop a customer from ordering something different and taking a chance that it will work.
For anything more than this, you should get legal assistance. Some locations place limits on what consumer rights can be waived, so that putting a “non-guarantee” in a contract might not actually be enforceable.