If they have overestimated how far the CPI is off, Boskin and his commission may institutionalize an underestimated CPI--guaranteeing a yearly, stealth tax increase.
( ( If ( they ( have ( overestimated ( ( how far ) ( ( the CPI ) ( is off ) ) ) ) ) ) ) ( , ( ( ( Boskin and ) ( his commission ) ) ( ( may ( institutionalize ( ( an ( underestimated CPI ) ) ( -- ( guaranteeing ( a ( yearly ( , ( stealth ( tax increase ) ) ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (IN If) (S (NP (PRP they)) (VP (VBP have) (VP (VBN overestimated) (SBAR (WHADVP (WRB how) (RB far)) (S (NP (DT the) (NNP CPI)) (VP (VBZ is) (ADJP (RP off))))))))) (, ,) (NP (NP (NNP Boskin)) (CC and) (NP (PRP$ his) (NN commission))) (VP (MD may) (VP (VB institutionalize) (S (NP (DT an) (NNP underestimated) (NNP CPI)) (: --) (VP (VBG guaranteeing) (NP (DT a) (JJ yearly) (, ,) (JJ stealth) (NN tax) (NN increase)))))) (. .)))
If they've overestimated how far the CPI is off, it will have horrific consequences. 
( ( If ( they ( 've ( overestimated ( ( how far ) ( ( the CPI ) ( is off ) ) ) ) ) ) ) ( , ( it ( ( will ( have ( horrific consequences ) ) ) . ) ) ) )
(ROOT (S (SBAR (IN If) (S (NP (PRP they)) (VP (VBP 've) (VP (VBN overestimated) (SBAR (WHADVP (WRB how) (RB far)) (S (NP (DT the) (NNP CPI)) (VP (VBZ is) (ADJP (RP off))))))))) (, ,) (NP (PRP it)) (VP (MD will) (VP (VB have) (NP (JJ horrific) (NNS consequences)))) (. .)))