So lets clear up the misconceptions about DCB...
Deep cycles will, have been, and are currently used to start engines including big block motors without effort. If that battery has the CCA required by the engine it will spin it just as fast as a starting battery. Not as long perhaps, but as long as the CCA are present there is no difference since the starter cannot tell whether those CCA are coming from a starting, deep cycle or hybrid battery. The FACT is that all batteries at the beginning of their discharge curve start out the same. It is the battery design that alters the shape of the discharge curve. It is true that a starting battery is designed to deliver a high current for a specific length of time after which the voltage drops rapidly. At the other end of the spectrum is a deep cycle which will deliver a much lower current (test spec is usually 23 or 25 amps) for a long period (usually 2 - 3 hours for a group 24 - 27). That does not mean it will not start an engine. It also means that continued cranking on an "untuned" or "balky" engine that a deep cycle would not survive that treatment for any length of time. But it will not be harmed by a 3 - 5 second starter draw
Batteries such as the 2xpower Batteries plus (can't remember who the actual manufacturer is right now) DCB's are totally fine to be used as a starting battery. Mine have been flawless for 1+yr & 25,000mi. Based on the year the 1fz-fe CCA spec is 450-650CCA, most quality group 27 DCB's are rated at around 900CCA +/- 100 (the 2xpower 27 is rated over 950CCA), that is plenty to handle our engine.
So take the 'never used a DCB for starting' statement with a grain of salt.
It is MORE important to have all of your batteries the same type, capacity and in service date.
lets talk abut 2 things
"It is MORE important to have all of your batteries the same type, capacity and in service date " this is correct only if you don't have saprator then a different type (or the same type different date) will discharge on each other because of difference of internal resistance
If they are connected Via diode or separation system this is not relevant
Some of your data are being just publicity and you didn't take into account cost and pay-load weight