what is the biggest known functional distance between an enhancer and its target gene (TSS) in cis?
I haven't found 1.5 Mb, but 1 Megabase. This is the case for the shh gene in chicken, where the enhancer element is located 1 Mb upstream. The original article can be found here: 


"A long-range Shh enhancer regulates expression in the developing
limb and fin and is associated with preaxial polydactyly."


There are two reviews which are also interesting in this context:


When needles look like hay: How to find tissue-specific enhancers in
model organism genomes
Remote control of gene expression.