Leask has a device that performs that function.
I would think it to be more advantagous to delay enrichment to a preset boost level instead of a preset rpm. As you know boost onset is a different rpm in different gears and ambient temperatures. Something like 0.4bar might be a good preset for boost reading delay at the WUR.
A quick data log of A/F ratios throughout your operating rpm range will tell you when it hits under those conditions. You can count on greater lag when the intercooler heat soaks or while driving on a hot humid day.
A simple hobs switch will perform that function.
I am going to do this myself very soon and will post all the dyno runs.
RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8