You might like to post this question at http://www.horncups.com as the members of that forum include professional horn players who might have played a Holton 105 as well as an Alex 103, a Paxman 25, a Conn 8D, other Holtons etc.  I simply don't know how effect changing the lead pipe is on a Holton 105 and whether it really changes the horn characteristics from an Alex 103 for smaller ensemble work to a Conn 8D for an American Symphony Orchestra sound ... I've heard arguments that the bell, bore throughout the instrument and weight all impact the sound and playing characteristics.